Run-Off-Road Collisions

Description of Strategies

Objectives

The objectives for reducing the number of ROR fatality crashes are to

  • Keep vehicles from encroaching on the roadside,
  • Minimize the likelihood of crashing or overturning if the vehicle travels off the shoulder, and
  • Reduce the severity of crashes that occur.

The ideal objective of good roadway design is to keep the vehicle in the travel lane. A secondary but related objective for a vehicle that inadvertently crosses the edgeline is to allow it to recover safely before going beyond the shoulder (if present) or back onto the roadside. Motorists will not purposely move onto the shoulder unless they need to pull over to slow or stop their vehicle. However, errant vehicles will cross over onto the shoulder and then the roadside, ending in an ROR crash. The reasons for inadvertent roadside encroachments are varied and include avoiding a vehicle, object, or animal in the travel lane; inattentive driving due to distraction, fatigue, sleep, or drugs; the effects of weather on pavement conditions; and traveling too fast through a curve. Several roadway design factors can also increase the probability that a driver error will become an ROR crash: travel lanes that are too narrow, substandard curves, and unforgiving shoulders and roadsides. Specific strategies can be applied to deal with ROR crashes caused by these factors. If a motorist travels onto the roadside, the probability of a crash depends to some extent on the speed of the vehicle and the driver's experience and capabilities. However, for normal travel on higher-speed roads, the crash probability, and primarily its severity, depends more upon roadside features, such as the presence and location of fixed objects, shoulder drop-off, sideslopes, ditches, and trees. If the roadside is fairly flat without objects and the soil can support the vehicle tires, the probability of a serious crash is minimal (and in many cases the driver fully recovers and there is no reported ROR crash). Conversely, when there is a continuous line of different types of objects and features or when the soil produces "vehicle tripping," the probability of a serious crash is high. Therefore, there are strategies directed at reducing the number and density of possible hazardous roadside features or the proximity of these features to the traveled war.

The final objective, reducing the severity of the crash, can be met by changes in the design of roadside features (e.g., making roadside hardware more forgiving or modifying sideslopes to prevent rollovers) and by changes in the vehicle (e.g., better restraint systems or improved side protection) or by increased occupant use of available restraints. A combination of strategies appears appropriate, with increased use of restraints providing the greatest benefit. This discussion focuses on roadway-related improvements.

Exhibit V-1 lists the objectives and several related strategies to reduce the consequences of ROR crashes. Details of these strategies are covered below. This is not a comprehensive listing of all possible strategies to reduce ROR crashes. For example, roadway design or rehabilitation strategies (such as building wide lanes or adding lane width on entire systems or subsystems) or the use of positive guidance principles in new roadway design can clearly affect ROR crashes. However, these strategies are most likely used in the design phase for new facilities or rehabilitation of long sections of roadways and are often high-cost improvements. AASHTO has chosen to concentrate efforts in this guide on lower-cost strategies that can be implemented relatively quickly, including strategies that can be applied to "spots" on the roadway (e.g., lane widening on hazardous curves). With few exceptions, it is these lower-cost, quickly implementable strategies that are covered below.

EXHIBIT V-1
Emphasis Area Objectives and Strategies

Objectives

Strategies

15.1 A—Keep vehicles from encroaching on the roadside

15.1 A1—Install shoulder rumble strips (T)

15.1 A2—Install edgeline "profile marking," edgeline rumble strips or modified shoulder rumble strips on section with narrow or no paved shoulders (E)

15.1 A3—Install midlane rumble strips (E)

15.1 A4—Provide enhanced shoulder or in-lane delineation and marking for sharp curves (P/T/E)

15.1 A5—Provide improved highway geometry for horizontal curves (P)

15.1 A6—Provide enhanced pavement markings (T)

15.1 A7—Provide skid-resistant pavement surfaces

15.1 A8—Apply shoulder treatments

• Eliminate shoulder drop-offs (E)

• Widen and/or pave shoulders (P)

15.1 B—Minimize the likelihood of crashing into an object or overturning if the vehicle travels beyond the edge of the shoulder

15.1 B1—Design safer slopes and ditches to prevent rollovers (see "Improving Roadsides," page V-36) (P)

15.1 B2—Remove/relocate objects in hazardous locations (see "Improving Roadsides," page V-36) (P)

15.1 B3—Delineate trees or utility poles with retroreflective tape (E)

15.1 C—Reduce the severity of the crash

15.1 C1—Improve design of roadside hardware (e.g., bridge rails) (see "Improving Roadsides," page V-36) (T)

15.1 C2—Improve design and application of barrier and attenuation systems (see "Improving Roadsides," page V-36) (T)

Note: The following page explains (T), (E), and (P) demarcations.

Types of Strategies

The strategies in this ROR guide were identified from a number of sources, including the literature, contact with state and local agencies throughout the United States, and federal programs. Some of the strategies are widely used, while others are primarily an experimental idea of a single individual or agency. Some have been subjected to well-designed evaluations to prove their effectiveness. However, it was found that many strategies, including some that are widely used, have not been adequately evaluated.

The implication of the widely varying experience with these strategies, as well as of the range of knowledge about their effectiveness, is that the reader should be prepared to exercise caution in many cases before adopting a particular strategy for implementation. To help the reader, the strategies have been classified into three types, each identified by a letter:

  • Tried (T)—Those strategies that have been implemented in a number of locations and that may even be accepted as standards or standard approaches, but for which there have not been found valid evaluations. These strategies—while in frequent, or even general, use—should be applied with caution, carefully considering the attributes cited in the guide and relating them to the specific conditions for which they are being considered. Implementation can proceed with some degree of assurance that there is not likely to be a negative impact on safety and very likely to be a positive one. It is intended that as the experiences of implementation of these strategies continue under the AASHTO Strategic Highway Safety Plan initiative, appropriate evaluations will be conducted so that effectiveness information can be accumulated to provide better estimating power for the user and the strategy can be upgraded to a "proven" (P) one.
  • Experimental (E)—Those strategies that have been suggested and that at least one agency has considered sufficiently promising to try on a small scale in at least one location. These strategies should only be considered after the others have proven not to be appropriate or feasible. Even where they are considered, their implementation should initially occur using a very controlled and limited pilot study that includes a properly designed evaluation component. Only after careful testing and evaluations show the strategy to be effective should broader implementation be considered. It is intended that as the experiences of such pilot tests are accumulated from various state and local agencies, the aggregate experience can be used to further detail the attributes of this type of strategy so that it can be upgraded to a "proven" (P) one.
  • Proven (P)—Those strategies that have been used in one or more locations and for which properly designed evaluations have been conducted that show it to be effective. These strategies may be employed with a good degree of confidence, but any application can lead to results that vary significantly from those found in previous evaluations. The attributes of the strategies that are provided will help the user judge which strategy is the most appropriate for the particular situation.

Targeting the Objectives

The first objective, keeping vehicles on the roadway, addresses various means of communicating with the driver. However, other strategies for fulfilling this objective target highway design features that could contribute to a crash (e.g., shoulder drop-offs and pavement with low skid resistance).

The second objective, minimizing the likelihood of an ROR crash given an encroachment, uses strategies that focus on the highway, with more concentration devoted to nonfreeway facilities and especially to higher-speed rural roads. Higher-design facilities such as freeways typically have fairly wide shoulders and more forgiving, wider clear zones. Features within the clear zone are shielded from traffic by barriers and crash attenuation devices. However, there is an extensive system of mostly two-lane, rural, high-speed roadways that do not have these features. The crash data presented earlier show that this system is particularly vulnerable to ROR crashes and should be targeted for appropriate measures. Vehicle design, restraint features and usage, and design of roadside features are all valid targets associated with the third objective, reducing the severity of ROR crashes.

The largest part of the ROR crash problem is on two-lane, rural, high-speed roads. Therefore, most of the emphasis in the following discussion of strategies is oriented to this road class. This is not to imply that there is no ROR problem on suburban or urban streets. Many of the strategies included in this guide could be implemented on such roadways, since many suburban roadways have "near-rural" designs. However, the strategy may well be restricted on these streets and roads by restricted right-of-way and preexisting roadside conditions (e.g., curbs and sidewalks, utility poles adjacent to the travel lane, and bicycle paths) that will prevent implementation of strategies oriented to the "shoulder" or "roadside." The urban safety engineer will have to rely on strategies related to keeping the driver in the travel lane (e.g., enhanced pavement markings, roadside delineation, and skid-resistant pavement).

There are also many miles of rural two-lane roads that carry significant traffic at fairly high speeds that are also characterized by very limited rights-of-way. These conditions limit the range of strategy choices. Some experimental strategies (see definition above) have been included that are oriented to these types of roadways (e.g., profile marking, edgeline rumble strips, modified rumble strips for narrow shoulders, various pavement markings at horizontal curves, and the delineation of utility poles and trees). These same strategies might be considered for urban and suburban streets. As is emphasized below, these strategies are considered experimental, since no valid effectiveness evaluations have been found. The user should not substitute these strategies for the "proven" strategies when the latter can be implemented.

Related Strategies for Creating a Truly Comprehensive Approach

The strategies listed above and described in detail below are those considered unique to this emphasis area. However, to create a truly comprehensive approach to the highway safety problems associated with this emphasis area, five types of related strategies should be included as candidates in any program planning process:

  • Public Information and Education (PI&E) Programs—Many highway safety programs can be effectively enhanced with a properly designed PI&E campaign. The traditional emphasis with PI&E campaigns in highway safety is to reach an audience across an entire jurisdiction or a significant part of it. However, there may be a reason to focus a PI&E campaign on a location-specific problem. While this is a relatively untried approach, as compared with areawide campaigns, use of roadside signs and other experimental methods may be tried on a pilot basis.

Within this guide, where the application of PI&E campaigns is deemed appropriate, it is usually in support of some other strategy. In such a case, the description for that strategy will suggest this possibility (see the attribute area for each strategy entitled "Associated Needs"). In some cases, specialized PI&E campaigns are deemed unique for the emphasis area and are detailed in the guide. In the future, additional guides may exclusively address the details regarding PI&E strategy design and implementation.

  • Enforcement of Traffic Laws—Well-designed and -operated law enforcement programs can have a significant effect on highway safety. It is well established, for instance, that an effective way to reduce crashes (and their severity) is to have jurisdictionwide programs that enforce an effective law against driving under the influence (DUI) or driving without seat belts. When that law is vigorously enforced with well-trained officers, the frequency and severity of highway crashes can be significantly reduced. This should be an important element in any comprehensive highway safety program.

Enforcement programs, by nature, are conducted at specific locations. The effect (e.g., lower speeds, greater use of seat belts, and reduced impaired driving) may occur at or near the specific location where the enforcement is applied. This effect can often be enhanced by coordinating the effort with an appropriate PI&E program. However, in many cases (e.g., speeding and seat belt usage), the impact is areawide or jurisdictionwide. The effect can be either positive (i.e., the desired reductions occur over a greater part of the system) or negative (i.e., the problem moves to another location as road users move to new routes where enforcement is not applied). Where it is not clear how the enforcement effort may impact behavior or where an innovative and untried method could be used, a pilot program is recommended. Within this guide, where the application of enforcement programs is deemed appropriate, it is often in support of some other strategy. Many of those strategies may be targeted at either a whole system or a specific location. In such cases, the description for that strategy will suggest this possibility (see the attribute area for each strategy entitled "Associated Needs for, or in Relation to, Support Services"). In some cases, where an enforcement program is deemed unique for the emphasis area, the strategy will be detailed. As additional guides are completed, they may detail the design and implementation of enforcement strategies.

  • Strategies to Improve Emergency Medical and Trauma System Services—Treatment of injured parties at highway crashes can significantly impact the level of severity and length of time during which an individual spends treatment. This is especially true when it comes to timely and appropriate treatment of severely injured persons. Thus, a basic part of a highway safety infrastructure is a well-based and comprehensive emergency care program. While the types of strategies included here are often thought of as simply support services, they can be critical to the success of a comprehensive highway safety program. Therefore, an effort should be made to determine whether there are improvements that can be made to this aspect of the system, especially for programs focused upon location-specific (e.g., corridors) or area-specific (e.g., rural areas) issues. Additional guides may detail the design and implementation of emergency medical system strategies.
  • Strategies Directed at Improving the Safety Management System—The management of the highway safety system is foundational to success. There should be a sound organizational structure, as well as infrastructure of laws, policies, etc., to monitor, control, direct, and administer a comprehensive approach to highway safety. A comprehensive program should not be limited to one jurisdiction, such as a state department of transportation (DOT). Local agencies often must deal with most of the road system and its related safety problems and are more familiar with its problems. Additional guides may detail the design and implementation of strategies for improving safety management systems.
  • Strategies that Are Detailed in Other Emphasis Area Guides—Any program targeted at the safety problem covered in this emphasis area should be created having given due consideration to the inclusion of other applicable strategies covered in the following guides:
  • Trees in Hazardous Locations,
  • Head-On Crashes,
  • Utility Pole Crashes (work in progress), and
  • Crashes on Horizontal Curves (work in progress).

Objective 15.1 A—Keep Vehicles from Encroaching on the Roadside

Strategy 15.1 A1—Shoulder Rumble Strips

General Description

EXHIBIT V-2
Shoulder Rumble Strips

Shoulder rumble strips are crosswise grooves in the road shoulder (Exhibit V-2). States have developed various design dimensions, but generally they are about 0.5 inches deep, spaced about 7 inches apart, and cut in groups of four or five. They can be rolled into hot asphalt or concrete as it is laid, or they can be milled in later. Vehicle tires passing over shoulder rumble strips produce a sudden rumbling sound and cause the vehicle to vibrate, thereby alerting inattentive, drowsy, or sleeping drivers of encroachment on the shoulder and possibly onto the roadside. Rumble strips have been used primarily on expressways and freeways, although some states install them on two-lane rural roads with a high number of single-vehicle crashes.

Many agencies have long used rumble strips on the roadway itself to alert drivers to unexpected or particularly important features ahead. Cross-lane rumble strips are commonly used in advance of stop signs on rural highways or in advance of construction zones. While the application of rumble strips on the shoulder is relatively new as an ROR safety strategy, there is considerable experience and information on design and construction, operational qualities, and the performance of shoulder rumble strips on freeways. Thus, this strategy is "proven"1 for such freeways. Due to the current use in some states, but lack of effectiveness information, shoulder rumble strips are considered a "tried" strategy for two-lane roads.

1 See explanation of ratings on page V-3.

Additional details concerning current practice with rumble strips can be found on the Federal Highway Administration's (FHWA's) "Rumble Strip Community of Practice" Web page at the following address: http://safety.fhwa.dot.gov/programs/rumble.htm. This site provides definitions of types of rumble strips used, detailed construction drawings, effectiveness estimates, and interviews with users and other experts, among other information. Information and details describing, for example, the three major types of rumble strips (milled, rolled [or formed], and raised) are given on the FHWA Web page.

Shoulder rumble strips are compatible with other strategies designed to reduce the likelihood or severity of roadside encroachments and can sometimes be implemented in the same project effort with appropriate planning at little or no additional cost (e.g., inclusion of rumble strips in safety-based shoulder reconstruction or curve-flattening efforts).

EXHIBIT V-3
Strategy Attributes for Shoulder Rumble Strips

Technical Attributes

Target

Drivers of errant vehicles, using sound and sensation to directly alert the individual of encroachment or pending encroachment.

Expected Effectiveness

On freeways, shoulder rumble strips have proven to be a very effective way to warn drivers that they are leaving or are about to leave the road. According to FHWA, several studies have estimated that rumble strips can reduce the rate of ROR crashes by 20 to 50 percent. Further statistics regarding effectiveness for specific programs are documented below. However, these crash reduction statistics apply to freeways.

While this strategy is currently implemented on nonfreeways by a number of jurisdictions, there is little information on the safety effectiveness of shoulder rumble strips on these roads. Further evaluation is clearly needed. Care should be taken in extrapolating freeway application experience to the two-lane highway system. On one hand, the rumble strips could be less effective since freeway design provides the errant driver with a wider clear zone in which to recover after hitting the strip. On many two-lane roads, the clear zone—often just a shoulder—is much more limited. In such cases, the driver has little opportunity to recover even when given a warning. However, rumble strips could be more effective on two-lane roads for basically the same reason: since two-lane roads have much less clear zone and much more hazardous roadsides (less breakaway objects, more severe sideslopes, objects closer to roadway), a higher proportion of excursions from the travel lane may become crashes. Moreover, the quality of the roadway alignment is generally worse on two-lane versus freeway facilities, and hence the need for such warning to keep drivers on the road is greater. Similarly, most freeways commonly include full 12-foot lanes, while there are many high-speed two-lane rural highways with lane widths as narrow as 10 feet. Thus, if the shoulder rumble strips are effective, they could prevent more crashes per excursion. While it is not possible to determine which set of assumptions is correct, shoulder rumble strips should produce measurable benefits somewhat consistent with those demonstrated in studies for freeways. In the absence of such information, the following studies provide effectiveness estimates for shoulder rumble strips on freeways and expressways.

The New York State Thruway Authority (NYSTA) installed continuous milled-in shoulder rumble strips on all four shoulders of 485 roadway miles of thruway between 1992 and 1993. In its before/after evaluation, NYSTA used accident data provided by the state police assigned specifically to the toll road system. One year of before data (1991) and 1 year of after data (1997) were used for the study (Exhibit V-4). Only single-vehicle ROR crashes with certain "causes" were selected for the study because "it was believed that these specific run-off-road crashes were indicative of those that could be mitigated by the use of continuous shoulder rumble strips and correcting the driver's behavior" (Perrillo, 1998). These causes included use of alcohol or drugs, driver inattention or inexperience, fatigue, illness, passenger distraction, and glare. Exhibit V-4 shows the reduction of crashes observed from 1991 to 1997.

In a companion study by the New York DOT of 300 miles of additional nonthruway mileage, the reduction of ROR crashes, resulting from driver inattention, fatigue, and drowsiness, is reported to be 65 percent with the installation of milled-in shoulder rumble strips (New York State DOT, 1998). The initial study also developed benefit-cost ratios for the rumble-strip installation program. The cost of installation was $3,995 per roadway mile for continuous rumble strips on all four paved shoulders. Hence, the total cost of installation for 485 roadway miles was more than $1.9 million. Using the cost of highway crashes as defined by the FHWA and assuming a yearly accident savings as summarized in Exhibit V-4, the total accident savings per year is $58.9 million. Assuming that the shoulder rumble strips have a maintenance-free lifespan of 6 years and that the yearly accident savings is as calculated by comparing 1991 data and 1997 data, the benefit-cost ratio equaled 186. Such a high benefit-cost ratio indicated shoulder rumble strips to be an extremely beneficial treatment.

In a recent study, the FHWA used data extracted from the Highway Safety Information System (HSIS) to study continuous rolled-in shoulder rumble strips installed on 284 miles of rural and urban freeway in Illinois and 122 miles in California. Where possible, the author used two different before/after methodologies, one involving "yoked" or paired comparison sites and one involving a nonpaired comparison group. In contrast with the more restricted group of accident types in the New York Thruway study, all single-vehicle ROR crashes were studied here. The Illinois data indicated an 18.3-percent reduction in single-vehicle ROR crashes on all freeways combined and a 13-percent reduction in single-vehicle ROR injury crashes. Both reductions were statistically significant. Comparable reductions on Illinois rural freeways were 21.1 percent for single-vehicle ROR crashes and 7.3 percent for injury crashes. California data for the combined urban and rural freeways indicated a 7.3-percent reduction in single-vehicle ROR crashes, but the finding was not statistically significant.

It is difficult to specify a crash reduction factor for shoulder rumblestrips on rural two-lane roads. There have been no effectiveness studies on such roads, and the effect could be logically hypothesized to be either less than or greater than on freeways. There are also differences in the estimated effects on freeways, with crash decreases ranging from 7 percent of total single-vehicle crashes to 90 percent of single-vehicle crashes related to driver inattention or fatigue. Part of this wide range is the result of differing crash types being studied (i.e., the more selective the crash type, as in the New York studies, the greater the effect will be). Part may also stem from effectiveness differences between milled-in rumble strips (in the New York studies) and rolled-in strips (in the FHWA study). However, no study has been identified that specifically addresses this potential difference in effectiveness. A "best guess" at this time might be a 20- to 30-percent reduction in single-vehicle ROR crashes on rural freeways, with less effect on urban freeways. For the reasons cited above, it is difficult to define even a "best guess" for two-lane rural roads. With no specific study on these roads, one might assume a similar effect to that seen on rural freeways—a 20- to 30-percent reduction in single-vehicle ROR crashes.

Keys to Success

If the use of shoulder rumble strips on freeways continues to be as effective as studies indicate, states should readily adopt them on these roads. The key to increased installation on two-lane and other nonfreeway roads would appear to be further proof of effectiveness on these roads and resolution of incompatibility issues such as bicycle use, noise, etc. (See "Potential Difficulties" below.) The use of prototype studies is suggested to establish the validity of extending this strategy to nonfreeway facilities. It will also be important to identify appropriate road sections—sites where ROR crashes are a problem and continuous shoulder rumble strips can be installed.

Potential Difficulties

Incompatibilities may exist between shoulder rumble strips and bicycle use. Since the transportation community encourages increased bicycle use, this may become a serious issue. In a recent Draft Technical Advisory on Roadway Shoulder Rumble Strips, FHWA has noted its full support of AASHTO's position, as stated in the 1999 AASHTO Guide for the Development of Bicycle Facilities, that

Rumble strips or raised pavement markers . . . are not recommended where shoulders are used by bicyclists unless there is a minimum clear path of 0.3 m (1 foot) from the rumble strip to the traveled way, 1.2 m (4 feet) from the rumble strip to the outside edge of paved shoulder, or 1.5 m (5 feet) to adjacent guardrail, curb or other obstacle. (Draft Technical Advisory on Roadway Shoulder Rumble Strips)

In that same advisory, the FHWA describes current state efforts to develop bicycle-friendly rumble strip programs and stresses the need for states to regularly sweep shoulders to remove debris where rumble strips and bicycles coincide in order to allow the bicyclists to use the outer rather than inner part of the paved shoulder.

It is also noted that the Association of Pedestrian and Bicycle Professionals (APBP) has commented on these guidelines (see http://www.apbp.org). Key suggestions for locations with bike traffic include only using rumble strips on two-lane roads where there is a significant, demonstrated crash problem (rather than a systemwide approach), minimizing the depth of the cut to 3/8 inch, preferably retaining 8 feet of clear paved shoulder outside the rumble strip, installing the strip at or under the edgeline rather than leaving the 1-foot "no man's land" between the edgeline and rumble strip, using 12-inch-wide strips with gaps, and no installation of rumble strips where there will be 4 feet or less of clear paved shoulder after installation without "overwhelming justification" and without warning signs to bicyclists.

In its early use of rumble strips, Pennsylvania would only use raised (edgeline) rumble strips where there was at least 4 feet of paved shoulder in order to accommodate bicycle use. The state required a minimum of 4 feet of paved shoulder for shoulder rumble strips and preferred 6 to 8 feet. Because of these concerns, Pennsylvania has developed a design to make shoulder rumble strips "bicycle-tolerable." Working for the Pennsylvania DOT, the Pennsylvania Transportation Institute researched alternative designs to alert motorists without being disruptive to bicyclists. The resulting design, which is used on shoulders at least 6 feet wide, is a 3/8-inch-deep cut that is 5 inches wide with a 7-inch space between cuts. The rumble strips begin 6 inches off the edge of the pavement. The Transportation Institute also recommended a similar pattern, except with a 6-inch space between cuts for lower-speed roads. Research in Pennsylvania continues on an appropriate design for roadways with narrower shoulders (2 to 4 feet). (See Appendix 1 for detailed drawings.) Due to similar concerns, California DOT (Caltrans) tested the vibration, noise, and subjective comfort levels of 11 different rumble strip configurations using passenger cars, trucks, volunteer bicyclists and State Highway Patrol motorcyclists. Based upon a combination of results from the different tests, Caltrans adopted new standard rolled-in and milled-in rumble-strip designs for routes with bicycle usage. Where the shoulder is less than 5 feet wide, the policy allows for the use of raised/inverted profile thermoplastic traffic strips as the edgeline. See Exhibit V-5.

Note that a similar raised edgeline design was modified in Great Britain due to bicycle and motorcycle concerns. The raised ribs in the final design are approximately 1/4 inch high. Details can be found at http://www.roads.dft.gov.uk/roadnetwork/ditm/tal/signs/02_95/index.htm. Of course, discouraging bicycle use on roadways prone to ROR crashes may be the appropriate thing to do (or providing safer, separated bicycle facilities within the same general corridor). To the extent that shoulder rumble strips would be used in a site-specific versus systemwide basis, this apparent conflict may be manageable. At least one state noted that motorcyclists may not be able to recover as well from riding along a rumble strip as from a normal paved shoulder. However, testing by Caltrans involving a very small sample of four state highway patrol motorcyclists indicated that the motorcyclists had no problems traversing any of the designs tested.

Other potential pitfalls include complications with snow removal, shoulder maintenance requirements, and noise. With respect to adverse weather, ice and snow can collect in rumble strips. When the trapped water freezes, icy conditions may occur. However, the drainage designed for shoulders, as well as the speed, turbulence, and vibrations from passing vehicles, tends to knock the ice from the rumble strips. Continuous shoulder rumble strips also have proven to be an asset to truck drivers during inclement weather. The shoulder rumble strips aid in determining the edge of the roadway when low visibility makes it difficult to see painted roadway edges and markings. (Note, however, that North Carolina has found the raised/inverted profile edgelines do not tolerate snowplowing.)

With respect to maintenance, Pennsylvania has not noted any additional maintenance required for the rumble strips installed on interstates with shoulders in good condition. Neither Massachusetts nor New York has noted any degradation over the past 3 years. Indeed, in some user states, rumble strips have been shown to help snowplows find the edge of the travel lanes. While some states have expressed a concern that the installation of rumble strips might lead to pavement deterioration, the FHWA "Rumble Strip Community of Practice" Web page indicates that this does not occur with proper installation. Finally, with respect to degradation, Kansas is changing its rumble strip policy, which allowed rolled-in strips, to one requiring milled-in strips. This change is due to Kansas's observation that rolled-in strips have a tendency to "heal over" and reduce effectiveness over time.

There have been reports of noise complaints where shoulder rumble strips have been installed. New installations should acknowledge this concern and make provisions where necessary. Implementing a program of rumble strips systemwide should consider local sensitivities to maintain support for such a program.

Finally, there is not a crash-proven rumble strip design for two-lane roads without paved shoulders or with very narrow paved shoulders (e.g., 2 feet wide). This is a significant problem for some state agencies and many county and local agencies where most or all two-lane roads do not have paved shoulders. It is possible that the effectiveness of shoulder rumble strips may well be lessened from freeway experience, by poor or narrow shoulders that exist on many two-lane highways, so that even an "alerted" motorist might not be able to safely recover. However, given the numbers of such miles in the United States, there is clearly a need to test some potential designs. (See sections below concerning possible experimental strategies.)

Appropriate Measures and Data

Process measures of program effectiveness would include the number of miles of road or the number of hazardous locations where rumble strips are installed.

Impact measures include the number of ROR crashes reduced at these locations and the changes in total crashes. If possible, the impact measure should include potential "crash migration" (i.e., crashes occurring on downstream sections where rumble strips have not been applied, but where drowsy drivers may still be on the road) effects on adjacent roadways.

The advent of low-cost vehicle-sensing and recording devices might allow for the use of a surrogate measure based upon the number of encroachments onto the shoulder over a specific section of road (e.g., a curve). In addition to process and crash data, the agency should also collect information on acceptance by the public and by bicyclists and on any adverse noise problems for adjacent properties.

Associated Needs

There have been a few reports of people who mistook the sounds produced by the rumble strips as car trouble. A public information or education campaign, as well as standard installation, should eliminate such misinterpretations. However, current moves to standardized use on freeways may provide the most effective public training.

Organizational and Institutional Attributes

Organizational, Institutional, and Policy Issues

First, if the agency does not have a design policy for rumble strips that can be retrofitted to shoulders, one may need to be developed. Additionally, a policy regarding the types of two-lane road sections where placement is acceptable may be necessary. While many states have established specific design and placement policies for shoulder rumble strips on freeways and other access-controlled facilities, specific criteria for two-lane or other nonfreeway roads were much more limited. For example, Minnesota policy states that "Rumble strips can also be placed on the shoulders of two-lane roads at the discretion of the District." Since 1991, the Kansas DOT has had a policy requiring shoulder rumble strips to be included on all reconstruction or new construction projects with a full width (8- to 10-foot) shoulder. Such strips were also required if full-width shoulders were being overlaid with a minimum of 1 inch of asphalt. This policy primarily pertains to freeways and expressways since few two-lane rural roads have full-width shoulders. However, Kansas is installing the rumble strips on its "Super-Two" sections—sections with 12-foot lanes and full-width shoulders. Finally, rolled-in strips on asphalt pavements tend to deform over time, thus reducing the size of the cuts and lessening their effectiveness. Due to these problems with "healing" rolled-in strips, Kansas is now considering a revision of this policy, which would mandate milled-in rumble strips. Review of freeway-related policies from Connecticut, New Hampshire, New Jersey, Massachusetts, Maine, and Minnesota indicate that factors to be considered in such policies include bicycle accommodation/routes, minimum shoulder width where allowable, offset from edgeline, placement on or near bridge decks, use at intersections, speed limits, and other factors. Second, while this strategy is implemented by the state DOT, there is a clear need for the inclusion of bicycle transportation offices or groups to be involved early in the planning process for treatment of nonfreeways.

Issues Affecting Implementation Time

Shoulder rumble strip programs can be implemented quickly, certainly within a year of an agency deciding to proceed. They can be implemented as components of both new construction and rehabilitation projects.

Costs Involved

Due to increased installation and technological advances, the cost of continuous shoulder rumble strips has decreased over the years. For instance, in 1990, the New York DOT reported paying $6.18 per linear meter compared with $0.49 per linear meter in 1998. Specific cost of installation on the New York Thruway was reported to be $3,995 per roadway mile for rumble strips on all four shoulders. The cost includes milling in the rumble strips, sweeping and discarding excess asphalt, and maintaining and protecting traffic. The Pennsylvania DOT reports an average cost of $0.25 per foot or $2,640 per mile for the installation of milled-in rumble strips on the shoulders on both sides of two-lane roads. Incremental costs would be even less for rumble strips being implemented concurrently with reconstruction or resurfacing of a highway.

Training and Other Personnel Needs

There appear to be no special personnel needs for implementing this strategy. Either agency personnel or contractors could do the installation. The need for training will depend on whether the agency has been using retrofitted rumble strips on freeways or other roadways. If not, either agency personnel or contractor personnel will need to be trained in proper installation techniques.

Legislative Needs

There do not appear to be any special legislative needs.

Other Key Attributes

 

One benefit of shoulder rumble strips is that, unlike other safety measures whose effectiveness may decrease over time as their "novelty" wears off, rumble strips primarily affect only drowsy or other inattentive drivers. Concern has been expressed that if fatigue-related crashes are prevented on one section of roadway, the problem may be transferred to another section. While the FHWA attempted to examine this issue, no data have been found to support or dispel the theory. Such a possibility may be reduced by public education urging fatigued drivers (particularly those who ride over the rumble strips and recover their vehicle from running off the road) to stop and rest before continuing.

EXHIBIT V-4
Before and After Data for Selected Single-Vehicle ROR Crashes on the New York Thruway (Source: New York State Police)

EXHIBIT V-5
California Raised/Inverted Profiled Thermoplastic Edgeline

Information on Agencies or Organizations Currently Implementing this Strategy

Almost all states have experience with shoulder rumble strips on interstates and other freeways. Some states are beginning to use them on two-lane roads. For example, as described in Appendix 2, Maryland has installed shoulder rumble strips on a limited number of miles of two-lane highways. Pennsylvania is currently installing "edgeline rumble strips" on the edgelines of two-lane roads with 4-foot shoulders (see Appendix 1). As noted above, Pennsylvania, California, and Colorado DOTs have developed a "bicycle-tolerable" rumble strip for use on such roads. As described in Appendix 3, Kansas is currently changing its rumble strip policy to move to milled-in strips only and has used shoulder rumble strips on limited sections of "Super Two" roadways (i.e., two-lane roads with wider lanes and full shoulders).

Strategy 15.1 A2—Rumble Strips for Roads with Narrow or Unpaved Shoulders (Experimental Treatment)

Most agencies require fairly wide paved shoulders before rumble strips will be installed (e.g., at least 4 feet in Pennsylvania, and 6 to 8 feet preferably in Pennsylvania and other states). However, state and local agencies are often faced with locations having high ROR crashes and either no paved shoulder or a very limited paved shoulder. In many cases, these roads are also characterized by lower average daily traffic (ADT) and limited right-of-way. Thus, widening and paving a shoulder for use with rumble strips may not be a viable option.

Three experimental2 treatments that might be considered include a milled-in "edgeline" rumble strip design placed on the edgeline (see Exhibit V-6), the above-noted raised/inverted profile thermoplastic profile marker (edgeline) that was tested by California for use on shoulders of less than 5 feet in width, and a modified "standard" rumble strip design for use on narrow shoulders (e.g., 2-foot paved shoulders).

2 See explanation of ratings on page V-3.

 

EXHIBIT V-6
Typical Drawing Detail for Milled Edgeline Rumble Strips for Noninterstate and Nonexpressway Use on Roadways (Source: Pennsylvania DOT)

 

North Carolina tested the raised/inverted profile marking on a limited sample of about 40 to 50 miles of two-lane rural U.S. routes, but has not completed any effectiveness evaluation (Appendix 4). Pennsylvania is pilot testing the milled-in edgeline rumble strip on sections with 4-foot paved shoulders and hopes to move to narrower shoulders in the future. Since this design is on the edgeline, it could be tested on roadways with no paved shoulders. As shown in Exhibit V-6 above, the Pennsylvania milled-in design is approximately 7 inches apart, 5 inches wide, 0.25 inches deep, and the width of the edgeline (see Appendix 1 for contact information).

North Carolina has also initiated a test program on a 9-mile section of rural two-lane highway with narrow (i.e., 2-foot) paved shoulders. The rumble strip design being used is a modification of the "standard" milled-in design used in North Carolina. The 7-inch-wide milled cuts are
5/8 inches deep, are separated by 5 inches of unmilled pavement, and extend 12 inches out from the edgeline (Appendix 4).

A major potential problem with the raised/inverted profile edgeline is durability in areas where snowplowing is done. North Carolina has experienced this problem in its pilot test and is no longer using this design in areas where snowfall is expected. Local residents have also complained of noise problems. The potential problems with the milled-in design for shoulders would be the same—possible (but not proven) complications with snow removal, shoulder maintenance requirements, and noise. If bicyclists regularly use these roads with narrow shoulders, the same concerns would exist.

Finally, these treatments have either not been tested or are being pilot tested at this point. They should not be used in place of other nonexperimental treatments and should be pilot tested and evaluated before widespread use. The effectiveness of these designs is unknown and may be well be lessened from the estimates in the previous section, since these sections have poor or narrow shoulders where even an "alerted" motorist might not be able to safely recover. However, given the number of miles of such roads in the United States where ROR crashes do occur, pilot testing of these designs is clearly warranted.

Strategy 15.1 A3—Midlane Rumble Strips (Experimental Treatment)

Midlane rumble strips are an experimental3 treatment that might be pilot tested on roadways with no shoulders or narrow paved shoulders. (This treatment is untested at this point, should not be used in place of other nonexperimental treatments, and should be pilot tested and evaluated before widespread use.) Midlane rumble strips appear similar to shoulder rumble strips—crosswise grooves in the pavement, perhaps 0.5 inch deep, spaced about 4 inches apart, and cut in groups of four or five, but installed in the center of the travel lane versus on the edge of the shoulder. They can be rolled into hot asphalt or concrete as it is laid, or they can be milled in later. Details of shoulder rumble strips that could be considered for use midlane can be found on the FHWA rumble strip Web site at http://safety.fhwa.dot.gov/programs/rumble.htm.

3 See explanation of ratings on page V-3.

Midlane rumble strips have the same intent as shoulder rumble strips. When the driver tracks a path leading to an encroachment on the roadside, the rumble strip acts on the inside tire (as opposed to the outside tire for shoulder rumble strips) to alert the driver. Unlike shoulder rumble strips, midlane rumble strips would be compatible with bicycle use, but may be incompatible with motorcycle use. In addition, there is fear among some designers and safety engineers that the strip in the center of the lane may become an additional driver distraction. Since midlane rumble strips should also affect head-on crashes, they might be considered at locations with both an ROR and a head-on crash problem.

The major potential difficulty with this strategy would be public acceptance, particularly with motorcyclists. The California DOT had four police motorcyclists test steering and recovery capabilities on 11 different shoulder rumble strip configurations and found no problems. However, further testing by nonprofessional riders is needed. Midlane strips could have other adverse effects, including potential snow removal problems, additional lane maintenance costs, and noise. Snow removal and maintenance problems have not been found to be a major issue for shoulder rumbles strips (see discussion in "Potential Difficulties" for shoulder rumble strips in Exhibit V-3).

Clearly this is a new, experimental intervention that will give a different look and feel to the roadway. Thus, a public information/education program is necessary to explain the benefits of the treatment, and public and motorcycle groups should be included in early planning activities.

In summary, while there are potential problems with this experimental treatment, if successfully tested, evaluated, and documented, it could provide a new tool for preventing ROR crashes on roadways where shoulder rumble strips cannot be installed, and the treatment might provide additional benefits in terms of reduction of head-on crashes.

Strategy 15.1 A4—Enhanced Delineation of Sharp Curves

General Description

ROR crash risk on rural two-lane roads increases with degree of curvature. Given the knowledge that sharper curves result in more shoulder encroachments and crashes and given that the flattening of the curve may be too costly, the concept here is to provide drivers with a clear picture of the sharpness of the curve prior to curve entry, to "warn" drivers of the hazardous situation, or to cause drivers to decrease their speed prior to entering the curve (Exhibit V-7). The first and second could be done through improved shoulder delineation (e.g., chevrons or high-intensity chevrons, large arrow signs, or delineators on guardrails); by improved curve warning signs (e.g., warning signs with flashing beacons); or innovative on-pavement markings (e.g., warning arrows on the pavement prior to the curve). The speed-reduction treatment would also involve innovative pavement markings that create a sense of "danger" (e.g., transverse lines with decreasing spacing or edgelines that give the appearance of a narrowing lane width). As a last resort, one state installed transverse rumble strips on the traveled way prior to the hazardous curve.

EXHIBIT V-7
Enhanced Delineation of Sharp Curves
The goal is to produce a delineation system "that will produce uniform speeds and placement throughout the curve. It will negate the need for excessive braking in the curve, and the absence of a change in speed within the curve is a prime indication that the driver has correctly perceived road curvature. Also, it will minimize encroachments on the centerline and edgeline and thereby leave most of the vehicles driving in the center of the lane" (Jennings and Demetsky, 1983).

Strategy Attributes
As noted, this strategy involves some type of delineation or pavement marking aimed at providing precurve information or warning to the driver. The proposed treatments are low-cost, currently available devices and markings. Since the speed of a vehicle entering a curve is related to the speed of the vehicle before the curve, it is important to reduce speed on tangent sections prior to the curve. All treatments suggested here are oriented to that goal. Speed reduction should result from better driver judgment, from driver recognition of and reaction to a well-designed and effective warning, or from driver reaction to a "heightened danger" situation (e.g., pavement markings that create the optical illusion of acceleration even at a constant speed).

As noted below, the shoulder delineation treatments are considered "proven"4 strategies in terms of crash reduction, while the on-pavement treatments, aimed at warning the driver or increasing the visible level of hazard, have been "tried" by states, but are not considered "proven" in terms of crash reductions. This series of treatments is compatible with other ROR and head-on crash treatments and should not adversely affect other road users such as bicyclists or motorcyclists.

4 See explanation of ratings on page V-3.

EXHIBIT V-8
Strategy Attributes for Delineation of Sharp Curves

Technical Attributes

Target

Drivers of vehicles entering potentially hazardous curves.

Expected Effectiveness

At least limited evaluations of all three types of devices have been conducted. Based on these studies, well-placed shoulder delineators are a proven crash-reducing strategy, at least for roads with average or higher designs. The on-pavement treatments aimed at warning the driver or providing an increased sense of hazard have been evaluated in terms of speed reduction, but not crash reduction. The positive findings with respect to speed reductions would place these treatments in the "tried" category.

In a very well-designed early study of post-mounted delineators on rural two-lane curves, Foody and Taylor (1966) found them to reduce ROR crashes by 15 percent. In a more recent nonaccident study, the "curve following behavior" of drivers was studied before and after rural, two-lane curves were treated with different combinations of chevron signs, post-mounted delineation, and raised pavement markings. Vehicle speed and the placement of the vehicle in the lane were measured at 46 sites in Georgia and 5 in New Mexico. The results for nighttime hours show that vehicles moved away from the centerline when chevrons were used (i.e., closer to the edgeline) and even farther away when raised pavement markers were used. When post-mounted delineators were used, vehicle placement on right curves shifted toward the centerline (Zador et al., 1987).

Contrasting findings for raised reflector posts were found in a Swedish study by Kallberg (1993). (Note that this study was not restricted to posts on curves.) The author concluded that "reflector posts on narrow, curvy, and hilly roads can significantly increase driving speeds and accidents in darkness." Specifically, reflector posts increased accidents on roads with relatively low geometric standards and 50-mph posted speed limits. Although the specific effects of reflector posts on the lateral position remain unclear, it is clear that the shift in lateral position (if there is a significant shift) is toward the edge of the road. This before and after study with control sites was conducted on roadway segments in Finland. The counterintuitive findings are supported by the human factors concept of selective visual degradation. This theory explains that reflector posts do not improve the driver's ability to detect potential hazards but do improve the driver's ability for orientation tasks. This may reduce the frequency of ROR collisions, but it also may increase speeds and therefore increase the severity of those ROR crashes that do occur. With respect to warning messages placed on the pavement, the Insurance Institute for Highway Safety (IIHS) conducted a study for a single, very sharp curve (~90°) on a suburban two-lane secondary road in Northern Virginia with a posted speed limit of 35 mph (Retting and Farmer, 1998). The pavement marking consisted of the word "SLOW" in 8-foot-high white letters, a white 8-foot-high left curve arrow, and an 18-inch-wide white line perpendicular to the road at the beginning and end of the text/symbol. Results were based on before/after changes in mean speed, 90th-percentile speed, and percentage of vehicles exceeding 35 mph, 40 mph, and 45 mph, as compared with similar data from a nearby comparison curve that was not treated. The pavement marking was associated with a decrease in vehicle speed of 6 percent overall and 7 percent during daytime and late night periods.

The same pavement marking was used in a 1999 study at six sites in Pennsylvania (Retting, 1999). A before/after study of effects on vehicle speeds showed that these pavement markings had little effect on the average speed and the 85th-percentile speed. However, the 95th-percentile speed was reduced significantly. This year, the marking will be implemented at 200 sites statewide, and IIHS will again evaluate the effect.

Evaluations of markings on the pavement to slow drivers by heightening "apparent danger" have been conducted for a number of years both in the United States and internationally. In a 1979 study for the Ohio DOT, the effects of yellow-bar pavement markings installed perpendicular to the direction of travel were studied. There were "reported reductions in traffic speeds, most notably high speeds" resulting from the pavement markings installed prior to curves (Retting and Farmer, 1998). In a somewhat limited 1980 before/after study of one particularly hazardous curve on a rural two-lane road in Meade County, Kentucky, the treatment involved transverse lines of reflective tape in an ever-tightening pattern designed to slow a vehicle from 55 mph to 35 mph before entering the curve. The pattern consisted of 30 stripes with a total pattern length of 810 feet, designed to give the illusion of acceleration unless the driver slowed down. Daytime mean speeds decreased from 41.3 mph to 33.9 mph immediately after installation. The mean speed increased slightly to 34.9 mph 6 months after installation. Nighttime mean speeds decreased from 40.5 mph to 35.1 mph immediately after installation and increased to 39.1 mph 6 months later. Average crashes per year decreased from 7.7 in the preceding 6 years to three crashes the year after installation. An estimated benefit-cost ratio of 45.9 was calculated, and the authors concluded that the treatment was more effective than signs alone and should be used at other curves where excessive speed is an accident factor (Agent, 1980). In a more recent study of "optical speed bars" at approaches to workzones, Meyer (2001) examined the issue of whether the decrease in speed from transverse striping was due to the perceptual effects of "increasing speed" with a pattern of stripes with gradually decreasing spacing or simply from the "warning" given. The author examined changes in speed as a free-flowing vehicle passed through three adjacent patterns when entering the work zone. The first pattern of transverse stripes were equally spaced, the second had ever-decreasing spacing, and the third were much wider stripes further apart. The data indicated speed reductions of 2 to 3 mph in the mean and 85th-percentile speeds for each of the patterns. The authors concluded that there was both "perceptual" and "warning" effects present. They drew no conclusions concerning which pattern was more effective. No crash data were analyzed. (The effects could differ between curves and work zones due to the driver's judgment of "hazard" related to each.)

In the earlier noted 1999 study of hazardous curve sites in Pennsylvania, transverse striping giving the illusion of acceleration was studied at several sites (Retting, 1999). Unlike the pavement arrow described above, the before/after study showed that these pavement markings had little effect on the average, 85th-percentile, or 95th-percentile speeds.

Other pavement markings designed to increase the "apparent danger" of the curvature have also been evaluated, but not for rural, two-lane curve situations. In a 1998 study of three urban exit ramps in Virginia and one ramp in New York, an experimental pavement-marking scheme was investigated. The treatment narrowed the apparent lane width of the entry to the ramp curve and the ramp curve itself by using a gradual inward taper of existing edgeline or exit gore pavement markings. Studies of vehicle speeds at three of the four ramps indicated that the proportion of passenger vehicles exceeding the posted speed limit by more than 10 mph decreased 20 to 30 percent while speeds at the control site and upstream site remained the same or increased. Similar or slightly larger decreases in the percentage of large trucks exceeding the posted advisory speed by more than 5 mph were also found at the three sites where the equipment differentiated trucks from other vehicles (Retting et al., 2000).

Finally, in the Netherlands and other European countries, an experimental use of edgelines has been tried on curves on narrow, low-volume roads where no edgeline was used in the past (Steyvers and Waard, 1997). Both a solid edgeline and a dashed edgeline caused vehicles to move away from the roadway edge when compared with a completely unmarked curve and with a curve with only a centerline. Driving speeds were slightly higher with the edgelines than with no lines, but slightly lower than when a centerline only was present. No crash analysis was conducted. While experimentation with such markings deserves further testing for these low-volume roads, current Manual of Uniform Traffic Control Devices (MUTCD) guidelines should be taken into consideration. In summary, there are few studies of the accident-related effects of these innovative treatments. Based upon the only crash studies available, post-mounted delineators might be expected to reduce ROR accidents on curves by approximately 15 percent. There is some question concerning the cost-effectiveness of continuous use of such devices on narrow, hilly, curvy roads with lower design standards. While warning symbols on the pavement prior to the curve, pavement markings "narrowing" the lane, and some transverse markings have been shown to reduce either mean speed or 95th-percentile speeds, there are no sound accident-based studies available. Thus, there continues to be a need for well-designed before/after pilot evaluations of crash experience, particularly for the pavement arrow and transverse striping treatments. The ongoing work in Pennsylvania should provide data on the arrow treatment.

Keys to Success

The development of design standards, based upon sound evaluation studies of these innovative markings, will be important. The ability of interested states to have access to evaluations in other states will be important to achieve acceptance.

Potential Difficulties

If these treatments are targeted to curves with actual or expected safety problems, there appear to be few potential difficulties. The Pennsylvania study of the initial transverse-bar sites noted some motorists driving on the shoulder to avoid the lines. This could be a problem with unpaved shoulders (but it is less likely to occur without paved shoulders) and if the vehicle makes a sudden avoidance maneuver without reducing speed (which, again, may not be likely to occur). Pennsylvania also noted that some drivers (presumably commuters) would drive across the centerline or onto the shoulders to avoid transverse rumble strips. Further observations of traffic behavior at treatment sites are needed to determine whether these are true problems. An attribute of these special treatments is their uniqueness and hence high level of notice by drivers. Overuse of these treatments could lead to them losing this uniqueness and ultimate effectiveness. A final possible difficulty could include maintaining the pavement markings over time, given that they are being crossed by all traffic.

Appropriate Measures and Data

In the evaluation of these delineation programs, process measures would include the number of hazardous curves treated.

Impact measures involve comparison of crash frequencies or rates (with the study appropriately designed) for the period before and after modifications. A useful surrogate measure is the change in speed for vehicles entering selected curves. The advent of low-cost vehicle-sensing and recording devices might also allow for the use of a surrogate measure based upon the number of encroachments onto the shoulder over a specific section of road (e.g., a curve). Sufficient data/information will be needed to target these treatments to the correct location. The expert system software noted in "Personnel and Other Training Needs" below will help in this effort.

Associated Needs Services

The transverse strips and the pavement arrow are new treatments, and a relatively modest public information effort may be helpful in garnering support for the effort. If evidence is found that a significant proportion of motorists do drive on the shoulder to avoid the transverse lines (see "Potential Difficulties" above) and if this is found to be a safety problem, then a more significant public education effort will be needed for this treatment.

Organizational and Institutional Attributes

Organizational, Institutional, and Policy Issues

These strategies will be implemented by state and local roadway agencies, and it does not appear that extra coordination with other agencies or groups is needed. If these treatments prove effective and are accepted by states for implementation, both specific design policies and placement policies will be needed. There are two different approaches in selecting delineators for a curve—local practice/policy and the MUTCD. Some of the "newer" pavement markings may have to be approved for use by FHWA as an experimental marking and then eventually adopted as an acceptable device for the MUTCD. However, until a standard is adopted, engineers should consider the effects of implementation inconsistencies on violating driver expectancies.

Jennings and Demetsky (1983) investigated the three post-mounted delineator systems used in Virginia (chevron, special striped delineator [on post], and reflector on a post) for their effectiveness in controlling ROR crashes and to recommend a standard policy regarding use of the system. The resulting simplified policy states that for moderate curves (less than 7 degrees) where delineation is necessary, standard delineation should be used as recommended in the MUTCD. If the curve is greater than 7 degrees, chevrons give better delineation information and the spacing should be 2 to 3 times MUTCD recommendation. Other policy-related advice on delineation selection and placement can be found in expert system software developed by Zwahlen and Schnell (1995). (See "Training and Other Personnel Needs" below.)

Issues Affecting Implementation Time

Since these devices are relatively inexpensive and standard, they could be implemented very quickly.

Costs Involved

The cost of the arrow pavement marker is about $2,000 per site (both directions) according to Pennsylvania's experience. Cost figures are not available for the other treatments. However, many states already use chevrons and other delineators in certain locations and may have cost figures of their own.

Training and Other Personnel Needs

There appear to be no special personnel needs for implementing this strategy. Either agency personnel or contractors would do the installation.

Since there are various low-cost devices available to the engineer, there is need for some guidance on treatment design and placement. Zwahlen and Schnell (1995) developed a PC-based expert system software package that helps the designer choose an appropriate treatment and place the devices for maximum effect. This expert system considers devices such as flexible post delineators, object markers, and various size chevrons.

Legislative Needs

None identified.

Other Key Attributes

 

None identified.

Information on Agencies or Organizations Currently Implementing this Strategy. As noted in the Effectiveness section, various states (e.g., Ohio, Virginia, Pennsylvania, Kentucky, and New York) have implemented limited installations of delineation and warning systems on curves. As documented in Appendix 1, the most recent of these is Pennsylvania, which is implementing and testing an innovative "pavement arrow" treatment on curve approaches.

Strategy 15.1 A5—Improved Highway Geometry for Horizontal Curves

General Description

Both ROR and head-on crashes are 1.5 to 4 times more likely to occur on curves than on tangents (Glennon et al., 1985). Zegeer et al. (1992) found that ROR crashes accounted for approximately 57 percent of the total crashes on a sample of over 11,000 curves on two-lane rural roads. While many of the other strategies in this section (e.g., rumble strips, shoulder treatments, wider clear zones, and skid-resistant pavement) would have equal or greater effectiveness on curves, crash reductions on curves can also be realized through tailored programs. Specifically, flattening curves (i.e., increasing the curve radius on two-lane rural roadways) has been found by Zegeer et al. to result in total curve crash reductions of up to 80 percent (i.e., flattening a 30-degree curve to 5 degrees). Thus, this strategy has been "proven"5 to reduce crashes. Given the size of these potential reductions, an agency should clearly consider this as a treatment alternative for locations with significant ROR problems if right-of-way and funding are available. Since some head-on crashes are the result of vehicles leaving the lane onto the shoulder area and then "overcorrecting" such that they cross into the opposing lane of travel and strike an oncoming vehicle, this treatment will also affect head-on crashes.

5 See explanation of ratings on page V-3.

Flattening of curves involves reconstructing a road section and changing the alignment. This strategy is among the higher-cost alternatives of those considered. Reconstruction may also entail the environmental process and will often include right-of-way acquisition, both of which require substantial time. Therefore, curve flattening will usually be outside the timeframe adopted for the AASHTO Strategic Highway Safety Plan. However, the strategy is included here since it can result in significant crash savings, which is based upon extensive research. Curve flattening is compatible with other ROR strategies such as shoulder or midlane rumble strips, enhanced delineation, wider shoulders, and roadside improvements. These complementary modifications, when implemented together, can result in lower costs than if they were instituted at separate times.

EXHIBIT V-9
Strategy Attributes for Improved Highway Geometry for Horizontal Curves

Technical Attributes

Target

While the treatment will target hazardous or potentially hazardous curves, the ultimate target is a vehicle that runs off the roadway on these curves.

Expected Effectiveness

Research by Zegeer et al. concerning this proven strategy provides estimates of the effect of curve flattening for various degrees of curve on two-lane rural roads (assuming that the central angle remains constant, and therefore the less-sharp treated curve will be longer and will "replace" some tangent in the initial layout). While more detailed estimates based upon type of curve (isolated versus nonisolated) and central angle (10 to 50 degrees) can be found in the full report, Exhibit V-10 indicates ranges of estimated percent reduction in total crashes for such treatments. For example, flattening a 30-degree curve to 10 degrees is predicted to reduce total crashes on the section by 61 to 67 percent. As noted in a recent review of this study and others, in work related to development of accident modification factors (AMFs) for use with FHWA's Interactive Highway Safety Design Model (Harwood et al., 2000), the estimates provided by this cross-sectional modeling effort would be expected to be less accurate than results from well-conducted before/after studies of actual curve flattening efforts. However, in the absence of such before/after studies in the literature, these results were accepted by the AMF expert panel.

As noted below, curve flattening along two-lane roads may be combined with other safety strategies, including lane and shoulder widening, to provide an additional safety benefit. Indeed, in the process of realigning a curve, the agency would simultaneously provide a new roadside, which itself could provide a positive contribution to safety. Exhibit V-10 summarizes the reductions possible. For instance, assume a 20-foot roadway (with two 10-foot lanes) is to be widened to 22 feet of paved surface with 8-foot gravel shoulders. Exhibit V-11 indicates that these improvements would reduce curve accidents by 5 percent (due to lane widening of 1 foot per side) and 24 percent (due to widening unpaved shoulders by 8 feet per side). Note that the 5-percent and 24-percent accident reduction values cannot merely be added numerically.

In summary, improving the geometry of horizontal curves can lead to significant crash reductions. These reductions change with the amount of curve flattening or widening, as shown in Exhibits V-10 and V-11. It is noted that these reductions are related to percentages of total crashes, rather than just to ROR crashes. While these treatments clearly affect ROR crashes, specific percentages for this subset are not presented in the study. The authors have noted that since curve flattening and widening affects almost all crash types, percent reduction in total crashes were considered to be the most appropriate measure.

Keys to Success

Since this is a relatively expensive treatment, one of the keys to success would appear to be targeting higher-hazard curves. Since ROR crashes increase with degree of curve, the targeting could be based primarily on prior crash history, curve degree, ADT, and speed limit.

Potential Difficulties

As noted above, the estimated effects of this treatment may be inflated due to the fact that they are not based on before/after studies. If the implementing agency "expects" effects this large for a given site or project and after-treatment experience is lower, the agency might curtail similar future efforts. However, given the size of the predicted effects, even if the true effects are much lower (e.g., half as high), this will still remain one of the most effective treatments for ROR crashes on curves.

Appropriate Measures and Data

In estimates of program implementation effectiveness, appropriate process measures would include the number or proportion of "hazardous" curves that are flattened (perhaps categorized by the change in curvature).

The impact measure would be the number of total crashes reduced in the roadway section replaced by the new design.

Targeting will require data on crash frequencies, degree of curve, length of curve, speed limit, and ADT. The factor most likely missing from computerized state files is the degree of curve.

Associated Needs

None identified. This is a standard treatment requiring no additional public information (except as part of any required environmental study).

Organizational and Institutional Attributes

Organizational, Institutional, and Policy Issues

This strategy will be implemented by the state DOT or local roadway agency, and it does not appear that coordination with other agencies will be needed. (The exception would be coordination with environmental agencies if new right-of-way were required.) Since curve flattening is a standard treatment, it would appear that new policy efforts are not required.

However, a slightly different "institutional safety philosophy" may be needed here in comparison with other strategies in this guide. Given the higher cost of this treatment (but coupled with the higher potential payoff), the agency must be prepared to implement more than just low-cost improvements.

Issues Affecting Implementation Time

Given that the treatment will require some form of design and reconstruction and will usually require purchase of additional right-of-way (and thus involve the environmental process), this treatment period will be relatively long.

Costs Involved

Costs will depend on the amount of reconstruction necessary and on whether additional right-of-way is required. In general, this is one of the higher-cost strategies recommended. It is also one of the most beneficial.

Training and Other Personnel Needs

There appear to be no special personnel or training needs for implementing this strategy, given that it involves "standard" reconstruction efforts.

Legislative Needs

None identified.

Other Key Attributes

 

Since curve flattening would require significant reconstruction, it would be very easy to combine this treatment with lane widening and shoulder improvement treatments noted elsewhere. In addition, it should provide some benefit for bicyclists using the shoulders since it reduces the number of vehicles that leave their lane.

 

EXHIBIT V-10
Percentage Reduction in Total Crashes on Two-Lane
Rural Roads Due to Curve Flattening (Based on Zegeer et al., 1992)

 

EXHIBIT V-11
Percentage Reduction in Total Crashes on
Two-Lane Rural Roads Due to Shoulder Widening (Based on Zegeer et al., 1992)

 

Strategy 15.1 A6—Enhanced Pavement Markings at Appropriate Locations

General Description
The focus of this strategy is the provision for better on-pavement "guidance" to drivers at locations where they might leave the roadway. This would be done through such alternative treatments as higher contrast or wider markings or raised pavement markers (RPMs) versus the standard pavement markings that would be used at other locations where the ROR risk is lower. (Note that this strategy relates to enhanced markings, often at spot locations, rather than to the installation of standard centerline and edgeline markings where no markings have existed in the past. The consensus of the literature on "standard" markings as reported in NCHRP Report 440 (Fitzpatrick et al., 2000) is that they are recommended for roadways with any substantial traffic volumes. Warrants for and details of standard centerline and edgeline markings can be found in the MUTCD (FHWA, 1988), and supplemental guidelines on implementation can be found in the Roadway Delineation Practices Handbook (Migletz, 1994).

Strategy Attributes

The goal of the strategy is to mark the roadway more clearly so that drivers will use the information to stay in their lanes and not merely to maintain or increase their speed. The specific markings to be used are low-cost, readily available materials.

As will be seen below, there remains conflicting evidence concerning the crash-related effectiveness of these devices. Thus, they are considered to be in the "tried"6 category of strategies. If truly effective, these treatments appear to be compatible with other ROR and head-on treatments and should not adversely affect other road users such as bicyclists or motorcyclists.

6 See explanation of ratings on page V-3.

EXHIBIT V-12
Strategy Attributes for Better Pavement Markings at Appropriate Locations

Technical Attributes

Target

Drivers of vehicles who might leave the roadway because of inability to see the edge of the pavement in the roadway section ahead.

Expected Effectiveness

Enhanced lane markings are an appropriate treatment if it is assumed that drivers leave the roadway because they cannot see the pavement edge in the downstream roadway sections. While some driver guidance is needed in such cases, the question is: How much should be added without changing the roadway geometry or the roadside design? Since some evaluations have raised questions about the overall effect of enhanced markings and RPMs, these features are considered a "tried" strategy at this time.

For example, past research (Pendleton, 1996) and research being conducted by Bellomo-McGee, Inc., for NCHRP indicate a lack of significant effect or even a possible increase in crashes on some locations. This could be because drivers tend to drive faster when presented with a clearer delineation of the lane edge. Note, however, that evaluations of such treatments reflect studies of projects involving delineation that was implemented in conjunction with resurfacing. What is not clear is whether speeds increase because of simultaneous resurfacing and remarking or because improved markings were added without alignment or shoulder treatments.

A review of earlier studies on wider edgelines in NCHRP Report 440 noted that, in general, the effectiveness of 8-inch edgelines to reduce ROR crashes is "questionable." The study recommends that they be used only on roads with 12-foot lanes, unpaved shoulders and ADT between 2,000 and 5,000 vehicles per day. In contrast, a 1988 study by the New York DOT indicated that sections of curving two-lane rural roads with new 8-inch edgelines resulted in higher crash reductions than similar sections with new 4-inch edgelines. The study indicated greater safety effects for total crashes (a 10-percent decrease for the wider edgelines versus a 5-percent increase for standard edgelines); for injury crashes (15-percent decrease versus 10-percent decrease, respectively); and for fixed-object crashes (33-percent decrease versus 17-percent decrease, respectively). The study appears to have controlled for the regression to the mean bias by choosing both sets of experimental and control sites from a listing of high-crash locations. It is not clear whether the choice was made randomly.

Effectiveness studies of RPMs have been conducted by states in before/after analyses of treatments at high-hazard locations. (It should be noted that accurately evaluating a treatment at a high-crash location is difficult because of the "regression to the mean" phenomenon. Whether the following studies controlled for such potential biases is unknown.) In southern New Jersey, RPMs have been used on two separate routes, both two-lane rural highways totaling 53.5 miles. The total project cost was $122,730 (1985 dollars). Using data from 2 years before and 1 year after, there was a statistically significant reduction in various types of nighttime accidents including total, injury, head-on, fixed object, overturn, and between intersection accidents. The calculated benefit-cost ratio was 19.89 (State of New Jersey, 1986).

In northern New Jersey, RPMs were installed on six routes (over 126 miles), generally rural two-lane roads. The total project cost was $314,242 (1985 dollars). Again, using data from 2 years before and 1 year after, there was a statistically significant reduction in various nighttime crashes including total, injury, property damage, overturn, head-on, fixed object, and between intersection crashes. The calculated benefit-cost ratio was 15.45 (State of New Jersey, 1986).

For projects with fewer than 800 markers, state forces (not independent contractors, as above) do the installation. For six different route sections totaling 47.8 miles, the construction cost was $151,493. Analysis results show a statistically significant reduction in accidents in every nighttime accident category (total, fatal, injury, property damage, head-on, fixed object, wet surface, and between intersections). The benefit-cost ratio was 25.51.

In Ohio, marker studies were conducted at 184 locations that had high accident rates prior to 1977, including horizontal curves, narrow bridges, stop approaches, and interchanges. Over 3,200 accidents at marker locations were analyzed 1 year before and 1 year after. The results show a 9.2-percent reduction in accidents and a 14.9-percent decrease in injuries. Markers were determined to be effective in all types of driving conditions, including nighttime (5.3-percent reduction) and adverse weather conditions (5.5-percent reduction in crashes at the same time precipitation increased by 10.6 percent). The study concluded that "a dollar spent on raised reflective highway markers in Ohio has returned $6.50 in savings due to accident reduction." As of 1981, nearly 700,000 RPMs were installed in Ohio (The Ohio Underwriter, 1981).

In a 1997 report, the New York State DOT concluded from prior evaluations that raised snowplowable pavement markers (RSPMs) can reduce "guidance-related accidents" (fixed-object collisions, ROR, and encroachment) by approximately 19 percent if selectively applied at locations having high percentages of such crashes. (It is not clear from the report whether the regression-to-the-mean bias has been accounted for.) Based upon an evaluation of 1992 data and a review of studies from other states, the DOT further concluded that RSPMs should not be applied systemwide, since they are somewhat costly and would have no effect or a possible negative effect on crashes at such nonspecific locations.

In summary, the effectiveness of RPMs as a general "systemwide" treatment appears questionable. The effectiveness of RPMs at high-hazard sites may also be less clear than first thought. This is not to say they should not be tried. Their relatively low cost argues for experimentation. However, at this point, it is not possible to specify a crash reduction factor for these devices. Clearly, well-designed before/after studies of effectiveness at such sites are needed—studies that account for the "regression-to-the-mean" bias. Thus, although this treatment may be effective in reducing crashes, careful targeting, monitoring, and evaluation are needed.

Similarly, the effectiveness of wider edgelines is also difficult to specify based upon past studies. While the NCHRP Report 440 review found wider edgelines "questionable" in general, the New York State DOT study indicated that implementation on high-crash sites on two-lane roads might result in a 10- to 15-percent decrease in ROR crashes.

Keys to Success

Based upon the effectiveness studies, the key to success is the targeted application of this treatment to sites where more guidance is needed for the driver, but where vehicle speeds will not be increased to unsafe levels.

Potential Difficulties

A potential difficulty with RPMs is the damage to the reflector or possible dislodging of the reflector during snow plowing. However, these concerns have lessened due to the creation of plowable RPMs. Another potential pitfall is nontargeted or erroneously targeted application of the devices on high-speed two-lane roads. This could result in adverse safety effects, which might negatively affect opinions about the treatment and therefore keep it from being implemented where needed.

Appropriate Measures and Data

In agency evaluations of implementation effectiveness, process measures would include the number of hazardous curves treated and the type of treatment applied.

Impact measures would involve before/after changes in crash frequencies or rates (with the study appropriately designed) and changes in speed from before to after treatment.

It would also appear that data are needed to better target the treatment, targeting to sites where additional visual guidance is needed, but where speeds are less likely to be increased. This is a difficult task. It may be aided by use of video logs and conduct of safety audit types of studies.

Associated Needs

No new public information efforts appear to be needed since this is a publicly accepted treatment on other roads. (Efforts to train the public to use them correctly—i.e., not to increase speed—are not expected to be effective).

Organizational and Institutional Attributes

Organizational, Institutional, and Policy Issues

This strategy could be implemented by the state DOT or a local roads agency, and it does not appear that additional cooperative efforts with other agencies are needed. The only exception might be if the enhanced delineation led to increased speeds. In this case, targeted speed enforcement could be needed.

After effectiveness is established and targeting methods are developed, a design and placement policy is needed to facilitate implementation, along with AASHTO support and guidance.

Issues Affecting Implementation Time

Since these devices are relatively inexpensive and are standard devices, they could be implemented in a very short timeframe.

Costs Involved

An old cost figure states that Ohio's average cost is $14.71 per unit for 35,000 units. A 1997 New York DOT report indicates that an RSPM (which is more expensive than a standard RPM) costs approximately $25­30 to install and $6­8 each 3 years for reflector replacement. Installation was found to increase the cost of delineation from approximately $2,000 to $5,300 per mile. However, states have most likely developed their own cost estimates, since these treatments are being widely used.

Training and Other Personnel Needs

There appear to be no special personnel or training needs for implementing this strategy. The installation would be done by either agency personnel or contractors and indeed is already being done in most state agencies.

Legislative Needs

None identified.

Other Key Attributes

 

None identified.

Strategy 15.1 A7—Skid-Resistant Pavements

General Description

EXHIBIT V-13
Distribution of Single-Vehicle ROR Fatalities for
Two-Lane, Undivided, Noninterchange,
Nonjunction Roads by Roadway Condition
(Source: 1999 FARS)
The 1999 statistics from FARS show that for two-lane, undivided, noninterchange, nonjunction roadways, 11 percent of single-vehicle ROR fatal crashes occur on wet roadways, with 3 percent more occurring on roadways with snow, slush, or ice (Exhibit V-13). Accidents on wet pavements are often related to the skid resistance of the pavement. It can also happen that the pavement friction available under dry roadway conditions will be significantly less than specified for the roadway and assumed in establishing design criteria (e.g., superelevation on curves). This can also lead to crashes. However, the major problem appears to be with wet pavement crashes.

A vehicle will skid during braking and maneuvering when frictional demand exceeds the friction force that can be developed at the tire-road interface. While this can happen on dry pavements at high speeds, friction force is greatly reduced by a wet pavement surface. In fact, a water film thickness of 0.002 inches reduces the tire pavement friction by 20 to 30 percent of the dry surface friction. Therefore, countermeasures should seek to increase the friction force at the tire-road interface and reduce water on the pavement surface. The coefficient of friction is most influenced by speed. However, many additional factors affect skid resistance, including the age of the pavement, pavement structural condition, traffic volume, road surface type and texture, aggregates used, pavement mix characteristics, tire conditions, and presence of surface water.

There has been a large amount of research funded by the FHWA, AASHTO, and pavement associations concerning designing better pavements—pavements which are more durable and more cost-effective (e.g., the FHWA/AASHTO Strategy Highway Research Program). The FHWA has issued a series of pavement-related technical advisories on such issues as needed changes in surface finishing of Portland cement concrete pavements for increased safety (FHWA, 1996). An important parameter in all this work is pavement skid resistance, perhaps the major safety-related factor along with pavement drainage design. However, most of this research and implementation effort is oriented toward policy or systemwide changes in new pavements or repaving efforts. While the best safety-related pavement design possible should be used in all paving efforts, the details of pavement design are beyond the scope of this guide.

Instead, this section will concentrate on improvements that can be made to sites that have, or are expected to experience, skidding-related ROR crashes. These usually involve improvements to increase skid resistance (higher friction factor). Such improvements should have high initial skid resistance, durability to retain skid resistance with time and traffic, and minimum decrease in skid resistance with increasing speed. Countermeasures to improve skid resistance include asphalt mixture (type and gradation of aggregate as well as asphalt content), pavement overlays on both concrete or asphalt pavements, and pavement grooving. Water can also build up on pavement surfaces due to tire rutting, an inadequate crown, and poor shoulder maintenance. These problems can also cause skidding crashes and should be treated when present. While there is only limited research on such site-specific programs (see below), the results of this research coupled with the results of research on the general effectiveness of decreasing skidding would place this in the "proven"7 category.

7 See explanation of ratings on page V-3.

EXHIBIT V-14
Strategy Attributes for Skid-Resistant Pavement Surfaces

Technical Attributes

Target

Treatment will target locations where skidding is determined to be a problem, in wet or dry conditions. The ultimate target, however, is a vehicle involved in a crash due to skidding, usually on wet pavement. With respect to ROR or head-on crashes, the target vehicle is one that runs (skids) off the road due to insufficient skid resistance or becomes involved in a head-on crash either by skidding into the opposing lane or by crossing into the opposing lane after an over-correction from an initial ROR maneuver caused by insufficient skid resistance.

Expected Effectiveness

There are many different specific countermeasures that may be implemented to improve skid resistance. This may include changes to the pavement aggregates, adding overlays, or adding texture to the pavement surface. The effectiveness of the countermeasure not only depends on that measure selected, but also will vary with respect to location, traffic volume, rainfall propensity, road geometry, temperature, pavement structure, etc.

The New York State DOT has implemented a program that identifies sites statewide that have a low skid resistance and treats them with overlays or microsurfacing as part of the maintenance program. A site is eligible for treatment if its 2-year wet accident proportion is 50 percent higher than the average wet accident proportion for roads in the same county. Between 1995 and 1997, 36 sites were treated on Long Island, resulting in a reduction of more than 800 annually recurring wet road accidents. These results and others within the state support earlier findings that treatment of wet road accident locations result in reductions of 50 percent for wet road accidents and 20 percent for total accidents. While the reductions in ROR or head-on crashes cannot be extracted from the data at this time, it appears that reductions in these types would be at least the same as for total crashes.

While these results could be subject to some regression-to-the-mean bias, the New York staff has found that untreated sites continue to stay on the listing until treated in many cases—an indication that these reductions are clearly not totally due to regression. The New York State DOT is planning a more refined data analysis to account for possible biases in these effectiveness estimates. Based on the current knowledge, this identification/treatment strategy would be classified as "proven."

Keys to Success

Monitoring the skid resistance of pavement requires incremental checks of pavement conditions. Evaluation must identify ruts and the occurrence of polishing. Recent research (Galal et al., 1999) has suggested that the surface should be restored between 5 and 10 years in order to retain surface friction, but the life span is affected by site characteristics such as traffic volume.

In addition, spot- or section-related skid accident reduction programs will be clearly most successful if targeted well. The New York State DOT program noted above provides a methodology for such targeting. In addition, in a 1980 Technical Advisory, the FHWA provided a detailed description of a "Skid Accident Reduction Program," including not only details of various treatments, but also the use of crashes and rainfall data in targeting the treatments.

Potential Difficulties

Skid resistance changes over time. This requires a dynamic program and strong commitment. As noted in the preceding section, it also requires good "targeting." When selecting sites for skid resistance programs, it is important to somehow control for the amount of wet-pavement exposure. This will help decrease the identification of sites that have a high wet-accident proportion or that rate simply because of high wet-weather exposure with no real pavement-friction problems. Unfortunately, it is difficult or impossible for an agency to develop good wet-pavement crash rates per vehicle mile for all roadway sections due to the lack of good wet-weather exposure data for all sites. Such data would require both good rainfall data for all potential sites and good measures of traffic volume during wet and dry weather. In its Skid Accident Reduction Program (SKARP), the New York State DOT uses a surrogate for such detailed data. The DOT compares the proportion of wet-weather crashes at each site with the proportion for similar roads in the same county. The assumption here is that rainfall (and thus wet-pavement exposure) would be similar across a county, a reasonable assumption.

Appropriate Measures and Data

Data are needed on traffic crashes by roadway condition. In addition, measures of traffic exposure that identify and reflect both dry and wet periods are needed. Finally, measurements of road friction and pavement water retention should be documented both before and after implementation of a strategy.

Associated Needs

None required. Relatively unnoticed by the public.

Organizational and Institutional Attributes

Organizational, Institutional, and Policy Issues

Implement by state DOT; no coordination required. Policy may be needed in order to determine the most appropriate pavement aggregate statewide and at special locations. Additionally, guidelines may be needed to highlight when pavement groove cuts should be considered. These countermeasures may also require cooperation within an agency, especially if these types of safety treatments are to be tied to routine maintenance.

Issues Affecting Implementation Time

Depends upon the treatment. Grooving can be done quickly, but overlays require more time. Nevertheless, all strategies being suggested should have short implementation periods.

Costs Involved

Highly variable depending upon the specific treatment. The New York State DOT estimates that its resurfacing/microsurfacing projects are approximately 0.5 miles long, with an average treatment cost of approximately $20,000 per lane mile (1995 dollars).

Training and Other Personnel Needs

No special personnel needs for implementing this strategy. Either agency personnel or contractors could do installation.

Legislative Needs

None identified.

Other Key Attributes

 

None identified.

Information on Current Knowledge Regarding Agencies or Organizations that Are Implementing This Strategy

Many states have an ongoing program aimed at the systematic monitoring of pavements, including measurement of skid numbers. As described in Appendix 5, New York State DOT has established SKARP to incorporate safety into pavement maintenance. The program was established to address problems with inadequate pavement friction. Using a systematic approach, over 100 statewide sites are identified annually and further tested and possibly treated. The treatments include resurfacing and microsurfacing.

Strategy 15.1 A8—Shoulder Treatments

General Description

If a vehicle that has intentionally or unintentionally left its lane and entered the shoulder area is allowed to safely recover, ROR crashes can be reduced. The probability of such a safe recovery is increased if the errant vehicle is provided with a wider and smoother area in which to initiate such a recovery and if the recovery is not impeded by a pavement irregularity that causes the driver to either fail to re-enter the lane or to enter it at such an angle that the vehicle crosses into the opposing lane. Shoulder treatments that promote safe recovery include shoulder widening, shoulder paving, and the reduction of pavement edgedrops (i.e., differences in lane pavement and shoulder surface heights, whether paved or not). While each strategy could be covered separately, the effectiveness is related, and the actual treatment can often be completed as a "package" during roadway resurfacing. Note that these same shoulder treatments, particularly shoulder paving and correcting edgedrops, can reduce head-on crashes. These treatments enable the vehicle's recovery to be made in a more controlled fashion and at a less sharp angle, thereby reducing the chances that the recovering vehicle will over-correct into the opposing lane.

Strategy Attributes

EXHIBIT V-15
Example of an Edgedrop
While the nature of the widening and paving treatments is self-evident, there are alternatives to the treatment of edgedrops (Exhibit V-15). Edgedrops can result from repaving, where material is added to the lane but not to the adjacent shoulder, or from weather or vehicle-caused "erosion" of unpaved shoulders. In their discussion of possible treatments, Humphreys and Parham (1994) noted that the best treatment is to always retain the lane and shoulder heights at the same level. This is often difficult due to repaving practices and unpaved shoulders that deteriorate. They then noted that an excellent alternative for both paved and unpaved shoulders is adding a 45-degree fillet at the lane/pavement edge: a wedge of pavement that would allow the vehicle to safely return to the roadway. This wedge (or a 45-degree beveled edge) can be added during repaving by attaching a device known as a "moulding shoe" to modern paving equipment.

Based upon current research concerning crash-related effectiveness, shoulder paving and widening are considered "proven" strategies, while edgedrop treatments are considered "experimental."8 These shoulder treatments are compatible with other ROR treatments. Paving shoulders can be accompanied by shoulder rumble strips, and paving and widening shoulders should make the shoulders more compatible with bicycle use.

8 See explanation of ratings on page V-3.

EXHIBIT V-16
Strategy Attributes for Shoulder Treatments

Technical Attributes

Target

The targets of this package of strategies are vehicles that stray from their lanes onto the shoulder area. The ultimate targets are the drivers of these vehicles, who are being provided with an opportunity for a safe recovery.

Expected Effectiveness

Even though there have been numerous studies of both shoulder widening and paving and limited studies of pavement edgedrop elimination, there is still some uncertainty about the true effect of such treatments. A recent unpublished literature review by Hauer (2000) demonstrated this uncertainty, noting some studies of shoulder widening/paving that indicated effects as large as 30- to 40-percent reductions and other studies that indicated no effect or even a possible increase in crashes for certain ADT levels. (If true, such an increase could be attributed to increased speeds resulting from shoulder improvements without changes in curvature or other factors.) The major shortcoming in the large body of research is that most findings are not based on well-conducted before/after studies where shoulders have actually been improved in the field. Instead, most are "cross-sectional" studies, in which different segments of roads with different shoulder characteristics are used in statistical models that estimate the effect of a change in width by changes in model output. However, based on the best available research, shoulder widening and paving would be considered "proven" strategies. Even though their safety benefits would appear to be "obvious," strategies related to edgedrop elimination would have to be considered "experimental," since no research into effectiveness is available.

With respect to shoulder widening and paving, in a recent FHWA effort related to determining AMFs for use with the Interactive Highway Safety Design Model (Harwood et al., 2000), a panel of experts attempted to develop a best estimate of shoulder treatment effectiveness based on a review of a number of research studies. Their estimate of effectiveness of shoulder widening on two-lane rural roads is shown in Exhibit V-17. Here, the base shoulder is a 6-foot-wide paved shoulder, and the AMFs shown for different ADTs are relative to this base shoulder. For example, a roadway with 500 vehicles per day and a 2-foot shoulder would be expected to have 30 percent more "related crashes" than the same road with a 6-foot shoulder (i.e., an AMF of 1.3). In like fashion, a two-lane rural road with 2,000 vehicles per day and an 8-foot shoulder would be expected to have 13 percent fewer related crashes than the same road with a 6-foot shoulder (i.e., an AMF of 0.87). Note that these reductions are not for total crashes, but for "related crashes," which include single-vehicle ROR, multivehicle opposite-direction (i.e., head-ons and opposing sideswipes), and multivehicle same-direction sideswipe crashes. To obtain the percentage reductions in total crashes, these AMFs would be multiplied by the percentage of total crashes they represent (typically, 35 percent for two-lane rural highways).

In the same study, the panel also defined AMFs for turf, composite, and stabilized gravel shoulders relative to the paved shoulder of the same width. As shown in Exhibit V-18, these effects change with shoulder type and shoulder width. For example, for an 8-foot width, turf shoulders are expected to experience 11 percent more "related crashes."

Much less is known about the effectiveness of edgedrop treatments, since it is difficult to specifically define the percentage of ROR or head-on crashes, which is the result of "overcorrection" by vehicles that run off the road first. Whatever that percentage, Humphreys and Parham (1994) concluded that a 45-degree-angle asphalt fillet at the lane edge would virtually eliminate this type of crash, even in cases where the shoulder is unpaved and suffers subsequent erosion damage.

Keys to Success

As with other ROR treatments, keys to success will include treatment targeting, such that funds are used as efficiently as possible. Targeting them to higher-speed roads with high ROR crash frequencies and rates could enhance all three strategies. Implementation of the edgedrop treatments will be enhanced by the identification of "champion" states that have implemented edgedrop treatments as a standard part of their repaving efforts and have found the treatments to be both low cost and effective. If an edge-fillet program is to be implemented, an additional key to success will be the development of an inclusive pavement specification and the necessary equipment modifications.

Potential Difficulties

While not evaluated extensively, it appears that the edge fillet or other edgedrop treatments would not have significant potential difficulties unless the use of this treatment resulted in less maintenance of unpaved shoulders.

However, if wider paved shoulders are added to high-speed roads with poor alignment and hazardous roadsides, they possibly could lead to an increase in vehicle speeds and total crash frequency and severity. Thus, careful targeting and monitoring is needed.

Appropriate Measures and Data

In the evaluation of strategy implementation effectiveness, process measures would include the number of road miles or number of hazardous locations where these shoulder treatments are installed, as well as the type of installation. Impact measures will include the number and rate of ROR (and head-on) crashes reduced at these locations. However, due to possible adverse effects, changes in total crashes also should be studied.

Data on ROR crashes would be needed to target the shoulder widening/paving treatment. If the state decided to use only the pavement edge treatment at selected locations (rather than as a standard add-on to resurfacing activities), criteria would need to be developed to define those critical locations, and data (e.g., crash or edgedrop inventory) would be needed to identify the locations. In addition, as noted above, since the edge-fillet treatment has not been evaluated, if a state were to implement the wedge, it is critical that the necessary treatment location, crash, and roadway inventory data on possible confounding factors be collected.

Associated Needs

Since these are somewhat "standard" treatments, there does not appear to be a critical need for public information or education efforts.

Organizational and Institutional Attributes

Organizational, Institutional, and Policy Issues

This strategy can be implemented by the state DOT or local roadway agency, and it would appear that there is no need for cooperative efforts with other agencies. Since these are "standard" treatments in general, no significant policy action appears needed other than a possible design policy for the pavement edge fillet.

Issues Affecting Implementation Time

Unless shoulder widening requires additional right-of-way, these treatments can be implemented in a relatively short timeframe. While all three would involve retrofits to existing pavements, it seems that the most opportune time to implement them would be in conjunction with repaving efforts.

Costs Involved

Shoulder widening costs would depend on whether new right-of-way is required and whether extensive roadside moderation is needed. Shoulder pavement costs should be similar to lane pavement costs and depend on how much shoulder stabilization is required.

Humphreys and Parham (1994) note that the cost of adding a pavement edge fillet when resurfacing a roadway is very low--perhaps 1 to 2 percent of the typical resurfacing cost.

Training and Other Personnel Needs

There would appear to be no special personnel needs for implementing these strategies, since they are similar to other paving/construction activities. The only new training needed would be for paving forces (whether state or contract) that would place the pavement edge fillet.

Legislative Needs

None identified.

Other Key Attributes

 

None identified.

 

EXHIBIT V-17
Accident Modification Factor for Paved Shoulder Width V-19 (Relative to 6-Foot Paved Shoulder) on Two-Lane Rural Highways
(Source: Harwood et al., 2000)

 

EXHIBIT V-18
Accident Modification Factor for Shoulder Type on Two-Lane Rural Highways
(Source: Harwood et al., 2000)

 

Information on Agencies or Organizations Currently Implementing this Strategy
Almost all states have some experience with widening and paving shoulders. The only state identified with a current policy regarding a pavement edge "wedge" is Kansas. The Kansas DOT has had a policy for more than a decade requiring that a pavement edge "wedge" be installed at the edge of pavement during 1-R (repaving) projects. Somewhat different from the 45-degree fillet of paving material described above, Kansas requires a wedge with a slope equaling the shoulder slope constructed of either rock, earth, or recycled asphalt. Rock is the most often used material. In all cases, the wedge material is compacted according to specifications.

Objective 15.1 B—Minimize the Likelihood of Crashing into an Object or Overturning if the Vehicle Travels Off the Shoulder

Strategy 15.1 B1—Design Safer Slopes and Ditches to Prevent Rollovers

This strategy has been covered below with closely related strategies under the section entitled Combined Strategy: Improving Roadsides.

Strategy 15.1 B2—Remove/Relocate Objects in Hazardous Locations

This strategy has been covered below with closely related strategies under the section entitled Combined Strategy: Improving Roadsides.

Strategy 15.1 B3—Delineation of Roadside Objects (Experimental Treatment)

This strategy has been covered below with closely related strategies under the section entitled Combined Strategy: Improving Roadsides.

Objective 15.1 C—Reduce the Severity of the Crash

Strategy 15.1 C1—Improve Design of Roadside Hardware (e.g., Bridge Rails)

This strategy has been covered below with closely related strategies under the section entitled Combined Strategy: Improving Roadsides.

Strategy 15.1 C2—Improve Design and Application of Barrier and Attenuation Systems

This strategy has been covered below with closely related strategies under the section entitled Combined Strategy: Improving Roadsides.

Combined Strategy: Improving Roadsides

The section includes strategies aimed at both minimizing the likelihood of a crash or overturning, if a vehicle travels off the shoulder and onto the roadside, and minimizing the severity of the crashes that do occur on the roadside. Because this strategy has multiple possible components that are covered in detail in other documents, the following narrative will be more general than the preceding sections. The reader should also refer to the AASHTO Roadside Design Guide for a detailed discussion of this strategy area.

General Description

The series of strategies covered first in this guide are related to keeping the vehicle from encroaching onto the shoulder—keeping the vehicle in its lane (e.g., shoulder rumble strips). The second set of strategies describing shoulder treatments is related to minimizing ROR crash likelihood by allowing the errant vehicle to safely recover to the travel lane. The set of strategies covered in this final section are related to the roadside—the area outside the shoulder. Each strategy is aimed at meeting one or both of the following goals:

  • Minimize the likelihood of crashing into an object or overturning if the vehicle travels beyond the edge of the paved (or unpaved) shoulder.
  • Reduce the severity of the crash if an impact occurs.

Improvements to the roadside can meet both goals by providing a traversable "clear zone," which (a) is free of highway hardware and unsafe natural objects (e.g., trees); (b) protects the objects that cannot be removed (e.g., crash attenuators in front of hazardous utility poles, guardrails protecting steep sideslopes); or (c) makes objects that cannot be removed less severe to the striking vehicle (e.g., breakaway signs and utility poles). In addition, the well-designed clear zone will

  • Be of sufficient width that most vehicles that leave the road do not exceed its limits,
  • Have up and down slopes that do not cause vehicle rollovers, and
  • Possess soil characteristics that do not lead to vehicle tripping and thus rollovers.

Strategy Attributes

Strategies that are directed at roadside design range from very costly to relatively inexpensive. The former include purchasing new right-of-way, building wider and safer clear zones where limited zones now exist, and clearing and grading clear zones on right-of-way already owned. Less costly (but not inexpensive) strategies may include replacing nonbreakaway or outdated roadside hardware (e.g., guardrail ends, culverts) with newer technology at selected locations, burying utility lines, or relocating utility poles. The question for the agency then becomes one of how to spend limited roadside safety dollars in the most cost-efficient manner. That is, what should be targeted, and how?

EXHIBIT V-19
Most Harmful Events for All 1999 Fatal Single-Vehicle ROR Crashes on All Roadway Classes
FARS data for all roadway classes shown in Exhibit V-19 indicate that the most harmful event in a nonintersection ROR crash is most likely to be an overturn (42 percent of 1999 ROR single-vehicle fatalities), an impact with a tree (26 percent), an impact with a utility pole (7 percent), or an impact with a ditch or embankment (5 percent). Most other roadside objects (e.g., culverts, posts, and guardrails) are found to be the most harmful event in 2 percent or less of the fatalities. FARS data shown earlier in Exhibit III-4 indicated that on two-lane rural roads, the most harmful event in a nonintersection ROR crash is most likely to be an overturn (41 percent of 1999 ROR single-vehicle fatalities), an impact with a tree (29 percent), an impact with a utility pole (8 percent), or an impact with a ditch or embankment (5 percent). Again, most other roadside objects (e.g., culverts, posts, and guardrails) are found to be the most harmful event in less than 2 percent of the fatalities. These are essentially the same percentages of most harmful events found when all single-vehicle ROR fatalities are examined, regardless of roadway class, as shown in Exhibit V-19. This is not surprising when one considers that approximately two-thirds of the total ROR fatality problem is on two-lane roads.

Rural and urban interstate roads (which experience approximately 18 percent of the ROR fatalities) exhibit a slightly different pattern, since the roadsides on interstates are built to a much higher standard. "Overturn" is much more prevalent as the most harmful event (59 percent); guardrails and concrete traffic barriers are slightly more prevalent (6 percent and 2 percent, respectively); ditch and embankments are slightly less prevalent (3 percent); and utility pole impacts are virtually eliminated. Somewhat surprisingly, while less prevalent, trees are still the most harmful event in interstate ROR fatal crashes in 13 percent of the cases.

Because of the importance of tree impacts on all roads and utility pole impacts on two-lane roads, they have been designated as separate emphasis areas with separate guides. For that reason, this section will not concentrate on these areas in terms of specific treatment strategies.

Rollover Reduction

If one were concerned with addressing a significant proportion of ROR fatalities with roadside treatments other than those aimed at trees and utility poles, the above data would point to concentration on treatments that lessen rollovers. The issue is complex, in that roadway and roadside design are only one of many factors affecting rollovers. Important factors not under the control of the roadway engineer include driver control factors (e.g., speed control or steering or braking during attempted recovery) and vehicle factors (e.g., the higher rollover propensity of SUVs and pickups). In addition, treatments aimed at rollover prevention are, in general, expensive. They either involve the development and deployment of new roadside hardware to replace outdated installations that might produce vehicle rollovers (e.g., replacing outdated guardrail ends) or, more often, widening and flattening roadside slopes. There is also some initial evidence that certain soils may cause rollovers on sideslopes that are otherwise safe. Given the size of the problem, and the severity of rollovers, the effort and expense is often warranted.

Because of the significance of the rollover problem, the FHWA initiated a major research effort in this area. While the results of that program will not be available until 2003, the initial phase of the effort—the experimental design of the research, which details the nature of the problem and possible avenues of research—has been completed (see Harkey et al., 2000). The roadside design features most likely to affect rollover include the sideslope (particularly fill slopes), ditch design, the nature of the soil on the slope, and the design of roadside hardware that might lead to rollovers (e.g., poorly designed guardrail ends). Unfortunately, little is known at this point concerning how soil types (or possible treatments) might affect rollover.

EXHIBIT V-20
Percentage of Subsequent Rollovers Related to Various "First Struck" Objects

With respect to roadside hardware, the 1999 FARS data also indicate that, when a rollover occurs as a "subsequent event," the first thing struck is a ditch or embankment in approximately 31 percent of the cases (Exhibit V-20). Other "first-struck" objects include trees (14 percent of the fatal crashes with subsequent rollovers), guardrail (11 percent), culverts (7 percent), utility poles (6 percent), and sign posts (3 percent). There is no one type of roadside hardware that is related to a large proportion of subsequent fatal rollovers: the roadside embankment and ditch design are the major problem. The fact that 11 percent of the fatal subsequent rollovers follow impacts with guardrails does not mean that the guardrails are poorly designed or substandard. Some of these rollovers could have occurred after the vehicle struck the rail and either passed over or through it. Alternatively, the crashes could have occurred when the vehicle rebounded from the rail, "struck" something else, and subsequently overturned. In addition, these statistics concern only the small percentage of crashes that result in a fatality and do not imply that 11 percent of all guardrail impacts result in rollovers. At most, these percentages suggest that there is not currently a guardrail design that will eliminate rollover under every set of operating conditions. While improvements in hardware is a worthwhile goal, the most effective rollover-reduction program will result from concentrating on "earth factors" and would involve widening and flattening sideslopes (particularly fill slopes) and making improvements to ditches. A related design issue concerning whether current standards that recommend a "length of need" for a guardrail are sufficient for today's conditions is currently under study by the FHWA--that is, whether the guardrail extends back far enough from the "hazard" to prevent vehicles from entering an unsafe sideslope or other hazardous condition.

The AASHTO Roadside Design Guide (2002) provides guidance concerning the recommended clear zone distance for given cut or fill slopes, design speed, and design ADT (Exhibits V-21 and V-22). The guide also presents adjustment figures based on horizontal curvature. As noted in the recent NCHRP Report 440 (Fitzpatrick et al., 2000), "The guidance curves provided [in the Roadside Design Guide] are based on limited empirical data that was then extrapolated to provide data for a wide range of roadside conditions; therefore, the numbers obtained from these curves represent a 'reasonable measure' of the degree of safety suggested for a particular roadway." Attempts are continually being made to update and improve these data. NCHRP Project 17-11, "Determination of Safe/Cost Effective Roadside Slopes and Associated Clear Distances," is aimed at increasing the understanding of roadside encroachment distances (i.e., the distance a vehicle strays from the travel lane) and rollover occurrence on roadside slopes.

 

EXHIBIT V-21
Clear Zone Distance Curves
(Source: AASHTO Roadside Design Guide, 2002)

 

EXHIBIT V-22
Clear Zone Distances (in Feet from Edge of Driving Lane)
(Source: AASHTO Roadside Design Guide, 2002)


 

Additional rollover (and other ROR) crash reduction could come from improved designs of roadside ditches. While the AASHTO Roadside Design Guide includes preferred foreslopes and backslopes for basic ditch configurations, these configurations are primarily seen on interstates and other higher-order roads and are often very different from the ditches on two-lane rural roads. Designs have not yet been developed to grade common two-lane shoulder-ditch combinations to reduce the chances of rollover and of vehicle encroachment across the ditch. Thus, based on today's knowledge, the best "rollover-prevention" program will be related to flattening and widening side slopes, particularly fill slopes.

In summary, based on research evidence to date, the set of strategies related to rollover (and ROR crash) reduction through changes in the "vertical" component of the sideslope (i.e., the degree and length or slope and related ditch design) would be considered "proven"9 strategies.

9 See explanation of ratings on page V-3.

Widening Clear Zones

Reducing rollover crashes is primarily accomplished by improvements to sideslopes and ditches. These are both components of the "clear zone"—the recovery area provided to vehicles that leave the roadway. In addition to modifying these "vertical" components of the clear zone, additional crash and fatality savings can be realized by modifying the width of the zone. The wider the object-free recovery area provided, the more likely an errant vehicle will either safely return to the travel lanes or stop on the roadside without a reportable crash. The width of the zone is normally set by either natural objects (e.g., trees, rock outcrops, embankments), or by roadside hardware (e.g., guardrails).

The question then becomes how wide to make the zone. While conventional wisdom (and the Roadside Design Guide) implies that a "safe clear zone width" on higher-speed roads is approximately 30 feet, there is no single width that defines maximum safety. Indeed, the guidance on clear zone width provided in the Roadside Design Guide is based on factors including design speed of the roadway, design ADT, the prevailing sideslope, and curvature. In general, the wider the better, up to some limit beyond which no significant number of vehicles will encroach.

Crash research provides a variety of approaches to answering the question of how wide a clear zone should be. For example, using crash, inventory, and ADT data from two states to study median widths (which act as "clear zones" for these freeways and other divided roads), Knuiman et al. (1993) found that accident rates continued to decrease as median widths increased up to about 80 feet. The effect was seen for head-on/opposite direction sideswipe crashes, as expected. A similar effect was also found for single- and multiple-vehicle crashes.

A 1995 study for the Texas DOT used a benefit-cost approach to establish guidelines for clear zone on suburban high-speed roadways with curb and gutter (Fambro et al., 1995). Based upon crash and roadside data and the ROADSIDE computer model, an appropriate and cost-beneficial clear zone width requirement was determined for such sections. "High-speed" was defined as a section with posted speed limits of 50 or 55 mph. The study focused on situations in which growth in traffic volume and frequency of turning movements necessitate the widening of an existing two-lane highway to four or more lanes. For this study, the baseline minimum clear zone width used in the calculation was approximately 10 feet. That is, even with added lanes, the existing right-of-way would allow at least 3.0 meters (10 feet) of clear zone. Based upon an incremental benefit-cost analysis for various combinations of baseline clear zone width, ADT, roadside hazard rating, and unit right-of-way acquisition cost, the study found the following in general:

  • It is not cost-beneficial to purchase 5 feet or less of additional right-of-way (given an existing minimum clear zone of 10 feet), since the relatively high fixed cost for relocation of utility poles is still present even for the relatively modest safety benefits seen with these small right-of-way acquisitions.
  • For unit right-of-way acquisition costs greater than $4 per square foot, it is not cost-beneficial to provide additional clear zone width through the purchase of additional right-of-way.
  • For roadways with a low roadside hazard rating, it is not cost-beneficial to provide additional clear zone width beyond the existing baseline clear zone width of 10 feet or more.

These findings are for suburban roads rather than rural roads, and the fatality costs used in the calculations were $500,000. Higher fatality costs such as those used today would change these break-even points.

In summary, the determination of an optimum clear zone width may best be answered in some type of economic analysis—comparing the cost of widening the zone with the savings in crashes (along with other costs and benefits). Savings in crashes will be a function of the number of vehicles that leave the roadway (which is strongly related to ADT, alignment, and vehicle speeds); how far they encroach onto the roadside (a function of exit angle, speed, and driver braking and steering); and the nature of the object that will be struck at the far edge of the clear zone. In short, this is a complex prediction problem.

An economic analysis program has been developed to aid the user in this effort: the ROADSIDE computer program. Details of the program are found in the AASHTO Roadside Design Guide. A revised and improved version of this program (the Roadside Safety Analysis Program, or RSAP) is expected to be completed by the end of 2002 under NCHRP Project 22-9 (Sicking et al., 2003). In addition, both these programs are based on limited data concerning the critical factors of roadside encroachment rates and extents; AASHTO is currently updating these data as part of the work in NCHRP Project 17-11, being conducted by the Texas Transportation Institute.

The wider the clear zone, the safer it will be. While additional guidance on widths and slopes and economic analysis techniques should be developed within the next 1 to 5 years, the best current guidance on widths and slopes is in the AASHTO Roadside Design Guide. Based on the current research concerning crash-related effectiveness, strategies aimed at widening the clearzone are considered to have "proven" effectiveness.10

10 See explanation of ratings on page V-3.

Improving Roadside Hardware and Natural Objects

The clear zone concept requires that no objects that can result in crashes be located in the zone. However, some objects must be located near the traveled way for a variety of reasons. These include hardware or objects related to traffic guidance or control (e.g., signs, some lighting supports); protection of more hazardous objects or situations (e.g., guardrails or median barriers); roadway design requirements (e.g., culverts); and traditional right-of-way uses (e.g., utility poles, mail boxes). Regardless of the reason, the best treatment for all objects is to remove them from the zone. If this cannot be done, alternative strategies include the following:

  • Relocating the objects either farther from the traffic flow or to less hazardous locations (e.g., relocating utility poles from the outside to the inside of horizontal curves).
  • Shielding or replacing "harder" objects with less hazardous breakaway devices (e.g., use of breakaway luminaire supports, or use of crash cushions in front of hazardous immovable objects).

The AASHTO Roadside Design Guide includes detailed discussion of this overall "forgiving roadside" strategy, along with design specifications, placement information and crash test results for a large number of roadside hardware devices. The guide also includes criteria for use in determining which of the many alternative hardware types should be chosen for a specific application.

A final strategy for improving roadside hardware involves replacing less forgiving, older hardware with newer designs. The Roadside Design Guide is also a useful reference in this context, since it provides effectiveness information on both older and newer hardware designs. For example, there is a detailed discussion of the Breakaway Cable Terminal (BCT) guardrail end, including problems that have been experienced due to improper installation (e.g., a lack of critical "flare" from the roadway). The guide presents information on a series of possible replacement terminals, including the Modified Eccentric Loader Terminal (MELT) and others. The guide does not include information on older guardrail terminal designs such as blunt or turned-down ends. The clear implication is that these designs are much less safe than newer designs and should be replaced.

The Roadside Design Guide provides general direction for a number of different types of hardware regarding when an older, outdated piece of hardware should be replaced: "This device should no longer be used on new installations for higher volumes and speeds, and should be upgraded as required by the state's policies and practices during rehabilitation projects or as needed during maintenance operations." More detailed guidance is given for roadside barriers. The primary criterion is whether the older barrier meets current structural guidelines (based primarily on crash test results) or whether it meets current design and location guidelines (e.g., too short to protect the hazard or too close to the hazard, based upon barrier deflection characteristics).

In a limited number of cases, the FHWA required that states upgrade older hardware. The most recent example involves the BCT and MELT guardrail terminals. The FHWA and AASHTO agreed to use only terminals that pass new crash test standards in new construction and rehabilitation projects as of October 1, 1998. Since neither the BCT nor MELT passed the new standards, neither can be used in new construction or reconstruction projects. Unfortunately, as will be seen in the later Effectiveness section, most of the upgrading guidance cannot be based on accident studies, since almost none exist. Instead, it is based solely on crash test results. For more information on barriers and other safety devices that have been approved by the FHWA for use on National Highway System roads, see http://safety.fhwa.dot.gov/fourthlevel/pro_res_road_nchrp350.htm.

In summary, based upon current crash-related research, the relocation of "hard" objects farther from the roadway or their replacement with more forgiving designs (e.g., breakaway designs) are considered "proven"11 strategies to reduce roadside harm. As will be seen below, the replacement of older "approved" barrier terminal designs with newer designs would be considered a "tried" strategy at this time, since sufficient real-world crash data have not been accumulated to move it into the "proven" category.

11 See explanation of ratings on page V-3.

Delineation of Roadside Objects (Experimental Treatment)

The above described strategies for reducing roadside crash risk and severity have been evaluated to some extend either through crash testing or crash-based evaluations. At least two states are currently pilot testing a low-cost experimental12 strategy where roadside objects are delineated so that they are more visible to drivers at night. Pennsylvania is testing this strategy at sites with high ROR utility-pole and tree-related crash frequencies (particularly at night) where it was not feasible to remove or relocate the object either due to budget constraints or due to the object being on private property. The tree or utility pole is marked with a 4-inch round of reflective tape. (One round is used on each tree and each utility pole, except for poles at intersections where two rounds are used.) The treatment is being tested in 11 districts in over 50 counties. (See Appendix 1 for contact information.)

12 See explanation of ratings on page V-3.

In the Iowa DOT pilot test, crash-prone utility poles that cannot be relocated or removed are being marked with a single band of 6-inch white reflective tape. Iowa identified corridors with high numbers of utility pole impacts from a statewide analysis and found that two 5- to 10-block corridors were in one city. One of the corridors was a four-lane street with higher speeds. The other included a 90-degree "bend" in the middle of the section. The Iowa DOT worked with the city (which owned the utility) in this effort. (See Appendix 6 for contact information.) They are now beginning to expand the pilot test to DOT districts.

The hypothesis is that such treatments could (a) provide additional guidance cues to the driver so that he or she is more likely to stay on the roadway (in the case of the regularly spaced utility poles); (b) make the hazard more visible (which might provide further "incentive" for staying on the roadway); or (c) provide "safer escape route" information to those vehicles that do leave the roadway (assuming the driver has time to react and to control the vehicle after leaving the roadway).

Pennsylvania has found that the utility pole tape can become detached and slide down the pole within a year in some cases. Iowa used an additional special adhesive to increase the life of the tape. Neither state has conducted what it considers to be a good evaluation of the utility pole tape, and so the utility pole tape's effectiveness is unknown. Therefore, the treatment must be considered experimental. It is unknown whether the additional visual input to the driver provided by these markings is beneficial or confusing. This should not be used in place of other nonexperimental treatment and should be pilot tested and evaluated before widespread use in any jurisdiction. (Note that these experimental strategies are not covered in Exhibit V-23.)

EXHIBIT V-23
Strategy Attributes for Roadside Improvements

Technical Attributes

Target

The targets for roadside improvement treatments are vehicles that leave the roadway, including those that return to the roadway out of control due to poor roadside design. However, the primary focus would be vehicles that strike objects on the roadside or overturn.

Expected Effectiveness

Three strategy areas have been covered for roadside improvements--rollover reduction due to flattening sideslopes, single-vehicle crash reduction due to flattening and widening sideslopes, and single-vehicle crash reduction and crash-severity reduction related to improvements in roadside hardware (e.g., replacing older hardware designs with newer designs). Historically, most roadside design and roadside hardware design improvements have been based on crash testing and recently on computer simulation. Due to the difficulty in collecting the necessary roadside inventory data to conduct a well-designed, accident-based study, few such studies exist. Thus, crash-based estimates of effectiveness are limited. Based upon these limited studies, the first two sets of strategies relating to sideslope and clear zone improvements are considered "proven" strategies. The replacement of older (approved) hardware with newer designs would be considered "tried" but not "proven" at this point due to the lack of crash-related evaluations. The following narrative describes some of the more important studies.

Zegeer, et al. (1987) examined the effects of sideslope on both rollover crashes and total single-vehicle crashes. They used field-measured crash, sideslope, cross section, and traffic data from approximately 1,800 miles on rural two-lane roads in three states. The rollover data were limited, making analysis of individual slope categories difficult. However, the authors found that rollover rates were significantly higher on slopes of 1:4 or steeper as compared with slopes of 1:5 or flatter. That is, in terms of rollover crashes, the 1:4 slopes were similar to the steeper 1:3 and 1:2 slopes. While the Roadway Design Guide would indicate the need for guardrail protection for 1:2 slopes, it would not for 1:3 slopes. The latter are generally considered to be "traversable ­ nonrecoverable," indicating that the vehicle would be expected to either stop on such a slope or continue to the bottom of the slope without overturning.

Based upon the same study (and a much larger sample size), it is concluded that single-vehicle ROR crashes (which include, but are not limited to, rollovers) can be significantly reduced by flattening existing sideslopes to 1:4 or flatter. As shown in Exhibit V-24, the estimated reduction in single-vehicle ROR crashes on two-lane rural roads ranges up to approximately 27 percent (i.e., for flattening a 1:2 slope to 1:7 or flatter). Because ROR crashes are a major component of total crashes on two-lane rural roads, and because flatter and safer sideslopes can decrease some head-on and sideswipe crashes due to safer recoveries, the corresponding decrease in total crashes for this example is an estimated 15 percent. These estimates are made under the assumption that the clear zone width stays the same and that the resulting sideslope is relatively free of rigid objects.

The Washington State DOT funded a study by Allaire et al. (1996) to determine whether past sideslope flattening projects had reduced ROR collision frequencies and severities. Unlike other studies, the authors were able to conduct a before/after study of the effects of slope flattening based upon a detailed review of 60 3R projects implemented in 1986­1991. Each of these 60 projects called for sideslope flattening in at least some portion of the project. The authors were not able to develop benefit estimates for specific degrees of flattening (e.g., flattening a 1:3 slope to 1:6) due to insufficient data on the precise "before" conditions. However, they were able to examine the before-to-after reductions in crashes by severity level for the treated sections and to compare these changes with a series of "control" changes. These comparisons included comparisons of actual ROR collision rate per mile (by severity level) in the after period with predicted after rates corrected for "other improvements" such as object removal and clear zone widening; predicted after rates based on the experience of the entire 3R project length, much of which did not include slope flattening; and predicted after rates based on changes in the statewide rate for similar roads during the same time period. In almost all cases, a statistically significant benefit of slope flattening was found. The percent reduction in ROR collision rates varied by comparison and by injury severity class from approximately 3 to 50 percent. Based upon examination of the tables, the estimated "median" reduction in ROR crash rate is approximately 25 to 45 percent.

Zegeer et al. (1987) also estimated the effects of clear zone widening on two-lane rural roads. If the existing recovery area measured from the edgeline is less than 10 to 15 feet, Exhibit V-25 presents the expected percentage reduction in "related crashes" (i.e., ROR, head-on, and sideswipe) due to clear zone widening by a given amount. For example, widening by 10 feet is predicted to result in a 25-percent reduction in these crashes.

With respect to removing roadside hardware from the clear zone or relocating it farther from the travel way, a study by Zegeer et al. (1990) developed the effectiveness estimates shown in Exhibit V-26 for two-lane rural roads. As can be seen, for example, moving culvert headwalls from 5 to 15 feet from the roadway would result in an expected 40-percent reduction in culvert headwall collisions on two-lane rural roads. Placing guardrails an additional 5 feet from the roadway would be expected to reduce the corresponding guardrail accidents by 53 percent. These estimates are based upon the assumptions that removal of a specific object leaves a wider clear zone and that other potentially hazardous objects do not remain at the same distance from the roadway. For example, if the culverts are at the edge of a row of large trees, then it is likely that culvert crashes will only be replaced by additional tree crashes.

The third strategy noted above involves the upgrading of existing roadside hardware. In a recent study, Ray (2000) examined the possible effects of upgrading guardrail terminals (e.g., BCT and MELT designs) to a newer design (the ET-2000) that does pass upgraded crash test standards. The author examined both past accident-based studies of the older designs in five states and recent data on the older and newer designs in three states. He used data from both police-reported and nonreported (maintenance) cases where available. He concluded that while the samples were small and the results varied greatly across the studies, he could detect no statistically significant difference in injury severity among the three designs for properly installed terminals. The author stressed the need for proper installation, since there is evidence that the BCT device was not installed properly (i.e., improper flare and offset from the travel lane) in a significant number of cases. And some data in earlier studies indicate that these improper installations are more hazardous (Morena and Schroeder, 1994; Agent and Pigman, 1991). Thus, one might expect an improvement from upgrading improperly installed BCT devices, for example.

Keys to Success

Keys to success would include accurate targeting; appropriate levels of funding (since most of these strategies can be relatively high cost); and a cooperative program among all agency divisions that can affect the roadside crash problem when a high-crash site is identified (e.g., traffic or safety engineering); during construction or reconstruction (e.g., roadway design division); or during normal maintenance operations (e.g., roadway maintenance forces). Appropriate targeting and analysis will require the development and regular use of methods for identifying sites with ROR crash problems related to the roadside.

Potential Difficulties

As noted above, simple clearing and grading to create small additions to the clear zone (e.g., 5 to 10 feet) on steeper sideslopes (e.g., 1:3 or 1:4) under high-speed conditions may not prove to be effective, since the vehicles entering the roadside may continue, due to the effect of the slope, to either overturn or traverse the clear zone and strike objects at its far edge. The other potential pitfall could be public reaction to tree cutting without appropriate public education, as well as coordination with environmental and other public groups.

Appropriate Measures and Data

The most appropriate measures will depend upon the strategy implemented, but would include process measures such as miles of roadside treated, number and type of hazardous objects removed or relocated, and number and type of older devices upgraded.

Impact measures must include both crash frequency or rate and crash severity, since some strategies will be successful even if only severity is affected.

Since targeting and site analysis appear to be keys to success, data needs would include accident, roadway and roadside inventory, and traffic data of sufficient accuracy and detail. The data most often missing are for the roadside inventory.

Associated Needs

As noted above, since tree clearing is a major component of some of these strategies, and since the public can view such tree removal negatively, there is a need for public information before tree-related strategies are employed. See Volume 3 of this report for more information.

Organizational and Institutional Attributes

Organizational, Institutional, and Policy Issues

While the primary agency would be the highway agency responsible for the right-of-way, certain strategies would clearly require participation of other agencies and public and private groups (e.g., strategies involving tree removal or utility pole relocation or removal). (See note on the need for a cooperative, multiagency program under "Keys to Success" above.) Since cooperation among various governmental and private groups is necessary if tree clearing is anticipated, see the more detailed discussion in Volume 3 of this report.

An organizational safety philosophy is needed that includes willingness to implement more than just low-cost improvements to optimize results. Many of these strategies are higher-cost strategies, but offer higher potential payoff.

Issues Affecting Implementation Time

The timeframe required will depend on the strategy chosen. It could be relatively short for treatments such as replacing older hardware at a specific location, but much longer if applied to an entire corridor or route system or if the treatment involved new right-of-way acquisition.

Costs Involved

Costs of these strategies can vary widely depending on the strategy chosen. Factors include whether new right-of-way is required or whether the treatment can be implemented as part of other rehabilitation or original construction efforts.

Training and Other Personnel Needs

Since most of these strategies are being implemented by many state highway agencies as part of construction or rehabilitation, there would appear to be no special personnel or training needs for implementing these strategies.

Legislative Needs

None identified.

Other Key Attributes

 

None identified.

 

EXHIBIT V-24
Percentage Reduction of Single-Vehicle and Total Crashes Due to Sideslope Flattening on Two-Lane Rural Roads (From Zegeer et al., 1987)

 

EXHIBIT V-25
Percent Reductions in "Related Accidents" Due to Increasing the Roadside Clear Recovery Distance on Two-Lane Rural Roads*

 

EXHIBIT V-26
Percent Reductions in Specific Types of Obstacle Accidents Due to Clearing/Relocating Obstacles Farther from the Roadway (Zegeer et al., 1990)

Targeting Roadside Improvements

Given the large number of miles of roadside and the expense of several of the important strategies (e.g., sideslope flattening or clear zone widening), it is important to target the various roadside improvement strategies to sites where they will be most beneficial. Targeting can be done in a number of ways, including the following:

  • Using existing ROR crash data.
  • Using computer programs like ROADSIDE and RSAP, which predict roadside crashes based upon roadway, roadside, and traffic descriptors.
  • Correcting a "corridor" based upon the fact that the hardware or clear zones do not meet current agency standards. For example, Washington State uses this third approach, along with more traditional "high-hazard location" treatments.

Initial location of potential treatment sites might be accomplished using programs that states currently use to identify hazardous sections of roads. These programs could be modified to focus on ROR crashes. These initial candidates might be further examined by comparing ROR rates at these sites with overall ROR rates on similar highway classes (e.g., two-lane rural roads). Care must be taken to avoid selection bias resulting from regression to the mean.

Since the ROR crashes being targeted here are those occurring on "problem" roadsides, further office-based examination of potential sites might then be accomplished by review of agency photologs or videologs for these sites. Lastly, since the final decision concerning treatment alternatives and final implementation should be based upon cost-effectiveness, the ROADSIDE or RSAP program could be used to examine the benefits and costs of alternatives. A study of sideslope flattening in Washington State (Allaire et al., 1996) concluded that the use of benefit-cost analyses of roadside safety improvements should be included in all types of highway construction projects to better identify how best to use roadside safety funds.

Other innovative targeting schemes could be used if an agency has some form of roadside inventory or roadside hardware inventory. In this case, the inventory data could be either used alone or combined with the crash data to target treatment locations. For example, if an inventory of guardrail terminals exists or can be collected from "windshield surveys" or photo/videologs, an agency could target terminal upgrading efforts to designs known to have problems if installed improperly (e.g., the BCT device). Colorado has an inventory of certain classes of hardware and has attempted to develop such procedures.

Compatibility with Other Strategies

All roadside improvement strategies would appear to be compatible with other ROR strategies aimed at keeping the vehicle on the roadway. The roadside improvement strategies are aimed at vehicles that leave the roadway even in the presence of other treatments. In addition, since these strategies affect areas outside the shoulders, they are compatible with bicycle and other uses. Widening existing clear zones, through the removal of trees, is perhaps both the most effective and the most problematic (it is problematic when there is opposition to removal of trees alongside roadways). The reader is referred to Volume 3 of this report for a full discussion on this potential conflict.

Information on Agencies or Organizations Currently Implementing this Strategy

Many state and local agencies implement clear zone policies for roadway construction. Some are implementing programs aimed at improving the clear zone, including programs focused upon flattening roadside slopes. Examples include the Washington State DOT 3R program, Washington's utility pole relocation program (which will be covered in an upcoming volume of this report), and Pennsylvania's tree removal program (see Volume 3 of this report). As required by FHWA, all states are using newer guardrail end terminal designs in new construction and reconstruction, and other hardware is often upgraded in major reconstruction projects. No states have been identified that are currently replacing older hardware on a systemwide basis. Finally, the Pennsylvania DOT is exploring the use of single-faced concrete barriers (instead of guardrail) in urban/suburban locations where the right-of-way is restricted and there is no option to relocate utility poles or to improve sideslopes.



Key References

AASHTO. Roadside Design Guide, 3rd Edition. Washington, D.C. (More information can be found at http://www.transportation.org/publications/bookstore.nsf/Home?OpenForm). 2002.

Agent, K. R. "Transverse Pavement Markings for Speed Control and Accident Reduction (Abridgement)." Transportation Research Record 773, Transportation Research Board of the National Academies, Washington, D.C. 1980.

Agent, K. R., and J. G. Pigman. Performance of Guardrail End Treatments in Traffic Accidents. Research Report KTC-91-1, Kentucky Transportation Center, University of Kentucky, Lexington, Kentucky. 1991.

Allaire, C., D. Ahner, M. Abarca, P. Adgar, and S. Long. Relationship Between Side Slope Conditions and Collision Records in Washington State. Final Report, WA-RD 425.1, Washington State Department of Transportation, Olympia, Washington. 1996.

Black, G. W., Jr., and L. E. Jackson. "Pavement Surface Water Phenomena and Traffic Safety." ITE Journal, pp. 32-37, February 2000.

Bray, J. Memo on "Skid Accident Reduction Program," NYSDOT. 2001.

Bucko, T. R., and A. Khorashadi. Evaluation of Milled-In Rumble Strips, Rolled-In Rumble Strips and Audible Edge Stripe. Office of Transportation Safety and Research, California Department of Transportation. 2001.

Fambro, D. B., R. L. Nowlin, S. P. Warren, K. A. Lienau, J. M. Mounce, R. P. Bligh, K. K. Mak, and H. E. Ross. Geometric Design Guidelines for Suburban High-Speed Curb and Gutter Roadways. Texas Transportation Institute, Texas A&M University, College Station, Texas. 1995.

FHWA. Draft Technical Advisory on Roadway Shoulder Rumble Strips. http://safety.fhwa.dot. gov/fourthlevel/rumstrp_ta.htm. 1999.

FHWA. http://safety.fhwa.dot.gov/rumblestrips/.

FHWA. "Skid Accident Reduction Program." FHWA Technical Advisory T 5040.17, December 23, 1980 (see http://www.fhwa.dot.gov/legsregs/directives/techadvs/t504017.htm).

FHWA. HSIS Summary Report: Safety Evaluation of Rolled- In Continuous Shoulder Rumble Strips Installed on Freeways. Washington, D.C. FHWA-RD-00-32. 1999.

FHWA. Surface Finishing of Portland Cement Concrete Pavement—Final Report FHWA-SA-96-068, Tire Pavement Noise and Safety Performance, May 1996. FHWA Policy Memorandum, Office of Engineering, November 12, 1996 (see http://wwwcf.fhwa.dot.gov/legsregs/directives/policy/sa_96_06.htm).

FHWA. Manual on Uniform Traffic Control Devices. Washington, D.C. 1988.

Fitzpatrick, K., K. Balke, D. W. Harwood, and I. B. Anderson. NCHRP Report 440: Accident Mitigation Guide for Congested Rural Two-Lane Highways. Transportation Research Board of the National Academies, Washington, D.C. 2000.

Foody, T. J., and W. C. Taylor. Curve Delineation and Accidents. Ohio Department of Highways, Bureau of Traffic. Columbus, Ohio. 1966.

Galal, A. A., R. Al-Mahrooqui, and R. Taha. Measurement, Analysis, Evaluation and Restoration of Skid Resistance on the Streets of Muscat. Transportation Research Board of the National Academies, TRB 78th Annual Meeting Preprint CD-ROM, paper #991343. 1999.

Glennon, J. C., T. R. Newman, and J. E. Leisch. Safety and Operational Considerations for Design of Rural Highway Curves, FHWA/RD-86/035. FHWA, U.S. Department of Transportation. 1985.

Griffith, M. S. Safety Evaluation of Continuous Shoulder Rumble Strips Installed on Freeways. Federal Highway Administration. Reference number: TRB No. 990162. 1999.

Harkey, D. L., F. M. Council, K. Digges, A. Eskandarian, W. W. Hunter, and K. K. Eccles. Effects of Highway Design on Rollover—Experimental Design. Federal Highway Administration, Washington, D.C. Unpublished report. 2000. Available from primary author at david_harkey@unc.edu.

Harwood, D. W. NCHRP Synthesis of Highway Practice 191: Use of Rumble Strips to Enhance Safety. Transportation Research Board of the National Academies. 1993.

Harwood, D. W., F. M. Council, E. Hauer, W. E. Hughes, and A. Vogt. Prediction of the Expected Safety Performance of Rural Two-Lane Highways, FHWA-RD-99-207, Federal Highway Administration. 2000.

Hatcher, C. W. "Grooving Streets and Highways Can Help Stop Skid Crashes." Traffic Engineering. 1974.

Hauer, E. Review of Literature Concerning the Safety Effects of Roadway and Intersection Factors. Unpublished. 2000. http://www.roadsafetyresearch.com/.

Humphreys, J. B., and J. A. Parham. The Elimination or Mitigation of Hazards Associated with Pavement Edge Drop-Offs During Roadway Resurfacing. AAA Foundation for Traffic Safety, Washington, D.C. 1994.

Jennings, B. E., and M. J. Demetsky. Evaluation of Curve Delineation Signs on Rural Highways. Virginia Highway and Transportation Research Council. 1983.

Kallberg, V. "Reflector Posts—Signs or Danger?" Transportation Research Record 1403. Transportation Research Board of the National Academies, Washington, D.C. 1993.

Knuiman, M. W., F. M. Council, and D. W. Reinfurt. "The Association of Median Width and Highway Accident Rates." Transportation Research Record 1401. Transportation Research Board of the National Academies, Washington, D.C. 1993.

Ligon, C. M., E. D. Carter, D. B. Joost, and W. W. Wolman. Effects of Shoulder Textured Treatments on Safety. Federal Highway Administration, Washington, D.C. FHWA/RD-85/027. 1985.

Meyer, E. "A New Look at Optical Speed Bars." ITE Journal. November 2001.

Migletz, J., J. K. Fish, and J. L. Graham. Roadway Delineation Practices Handbook. FHWA-SA-93-001, Federal Highway Administration, Washington, D.C. 1994.

Morena, D. A., and L. S. Schroeder. Accident Analysis of the Michigan Breakaway Cable Terminals (BCT). Federal Highway Administration, Michigan Division, Washington, D.C. 1994.

New York State Department of Transportation. 1988 Annual Evaluation Report: Highway Safety Improvement Program. 1988.

New York State Department of Transportation. Safe-Strips (Safety Shoulder Rumble Strips). April 1998.

New York State Department of Transportation. Raised Reflectorized Snowplowable Pavement Markers: A Report to the Governor. 1997.

The Ohio Underwriter. "Research on Reflective Markers Indicates Savings of $6.50 for each Dollar Spent." 1981.

Perrillo, K. The Effectiveness and Use of Continuous Shoulder Rumble Strips. Federal Highway Administration. 1998. http://safety.fhwa.dot.gov/rumblestrips/resources/rumblekp.htm.

Pendleton, O. Evaluation of Accident Analysis Methodology. Federal Highway Administration, Washington, D.C. FHWA RD-96-039. 1996.

Ray, M. H. "Safety Effectiveness of Upgrading Guardrail Terminals to NCHRP Report 350 Standards." Transportation Research Record 1720. Transportation Research Board of the National Academies, Washington, D.C. 2000.

Retting, R. A. Personal correspondence from author concerning Pennsylvania curve marking experiment. 1999.

Retting, R. A., and C. M. Farmer. "Use of Pavement Markings to Reduce Excessive Traffic Speeds on Hazardous Curves," ITE Journal. September 1998.

Retting, R. A., H. W. McGee, and C. M. Farmer. "Influence of Experimental Pavement Markings on Urban Freeway Exit-Ramp Traffic Speeds." Transportation Research Record 1705, Transportation Research Board of the National Academies, Washington, D.C. 2000.

Sicking, D. L., K. K. Mak, and K. Zimmerman. NCHRP Report 492: Roadside Safety Analysis Program (RSAP)—Engineer's Manual. Transportation Research Board of the National Academies, Washington, D.C. 2003.

State of New Jersey. Annual Safety Report. Fiscal Year 1986.

Steyvers, F. and D. Waard. "Road Edge Delineation in Rural Areas: Effects on Driving Behaviour," Proceedings of the Human Factors and Ergonomics Society, HFES Europe Chapter Annual Meeting, Haren, the Netherlands. 1997.

Torbic, D., L. Elefteriadou, M. El-Gindy. "Development of Rumble Strip Configurations that Are More Bicycle Friendly." Transportation Research Record 1773. Transportation Research Board of the National Academies. 2001.

Zador, P., H. S. Stein, P. Wright, and J. Hall. "Effects of Chevrons, Post-Mounted Delineators, and Raised Pavement Markers on Driver Behavior at Roadway Curves." Transportation Research Record 1114, Transportation Research Board of the National Academies. 1987.

Zegeer, C. V., J. R. Stewart, D. W. Reinfurt, F. M. Council, T. R. Newman, E. G. Hamilton, T. Miller, and W. W. Hunter. Cost Effective Geometric Improvements for Safety Upgrading of Horizontal Curves—Final Report. Federal Highway Administration, Washington, D.C. 1990.

Zegeer, C. V., J. Hummer, D. Reinfurt, L. Herf, W. Hunter. Safety Effects of Cross-Section Design for Two-Lane Roads—Volumes I and II. Federal Highway Administration, Washington, D.C. FHWA-RD-87-008. 1987.

Zegeer, C. V., J. R. Stewart, F. M. Council, D. W. Reinfurt, and E. Hamilton. "Safety Effects of Geometric Improvements on Horizontal Curves," Transportation Research Record 1356. Transportation Research Board of the National Academies. 1992.

Zwahlen, H. T., and T. Schnell. "Knowledge-Based Personal Computer Software Package for Applying and Placing Curve Delineation Devices." Transportation Research Record 1495. Transportation Research Board of the National Academies. Washington, D.C. 1995.