Horizontal Curves

Descriptions of Strategies

Objectives

The objectives for reducing the frequency and severity of curve-related crashes are to

• Reduce the likelihood of a vehicle leaving its lane and either crossing the roadway centerline or leaving the roadway at a horizontal curve and
• Minimize the adverse consequences of leaving the roadway at a horizontal curve.

Exhibit V-1 presents these objectives and their related strategies for improving safety at horizontal curves. Because the AASHTO Strategic Highway Safety Plan is geared toward low-cost, short-term safety improvements, the list of strategies presented in the exhibit is arranged from low-cost, short-term treatments to high-cost, long-term treatments.

Exhibit V-1
Objectives and Strategies for Improving Safety at Horizontal Curves

Objectives	Strategies
15.2 A Reduce the likelihood of a vehicle leaving its lane and either crossing the roadway centerline or leaving the roadway at a horizontal curve	15.2 A1 Provide advance warning of unexpected changes in horizontal alignment (T) 15.2 A2 Enhance delineation along the curve (T) 15.2 A3 Provide adequate sight distance (T) 15.2 A4 Install shoulder rumble strips (P) 15.2 A5 Install centerline rumble strips (T) 15.2 A6 Prevent edge dropoffs (T) 15.2 A7 Provide skid-resistant pavement surfaces (T) 15.2 A8 Provide grooved pavement (T) 15.2 A9 Provide lighting of the curve (T) 15.2 A10 Provide dynamic curve warning system (T) 15.2 A11 Widen the roadway (P) 15.2 A12 Improve or restore superelevation (P) 15.2 A13 Modify horizontal alignment (P) 15.2 A14 Install automated anti-icing systems (T) 15.2 A15 Prohibit/restrict trucks with very long semitrailers on roads with horizontal curves that cannot accommodate truck offtracking (T)
15.2 B Minimize the adverse consequences of leaving the roadway at a horizontal curve	15.2 B1 Design safer slopes and ditches to prevent rollovers (P) 15.2 B2 Remove/relocate objects in hazardous locations (P) 15.2 B3 Delineate roadside objects (E) 15.2 B4 Add or improve roadside hardware (T) 15.2 B5 Improve design and application of barrier and attenuation systems (T)
See the following section for explanation of (E), (P), and (T) designations.

Explanation of Strategy Types

The strategies in this 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 used at a state or even a local level. 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 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 in the AASHTO guides have been classified into three types, each identified by a letter:

• 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 users judge which strategy is the most appropriate for the particular situation. Within the proven strategies, several references are made to accident modification factors (AMFs). For a detailed description of how to use AMFs, see Publication No. FHWA-RD-99/207 entitled Prediction of the Expected Safety Performance of Rural Two-Lane Highways by Harwood et al. (2000).
• 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 valid evaluations have not been found. These strategies, while frequently or even generally used, should be applied with caution; users should carefully consider the attributes cited in the guide and relate them to the specific conditions for which they are being considered. There can be some degree of assurance that implementation will not likely have a negative impact on safety and will very likely have a positive one. In this context, effectiveness refers to the likelihood of a strategy reducing either the severity or the frequency of crashes. In some cases, the effectiveness of treatments may have been shown to have a desired impact on one or more presumed surrogates for crashes (e.g., speed), but the translation of that effect to actual crash experience has not yet been demonstrated. It is the intent that as these “tried” strategies are continually implemented 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” 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 be considered only after the others have been determined to be inappropriate or unfeasible. 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 evaluation 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” one.
  
  The classification of a traffic control device strategy as experimental means that the strategy has been used either on only a small scale or not at all. It is not meant to imply that the strategy has been approved for experimentation by the National Committee on Uniform Traffic Control Devices (http://www.ncutlo.org/news.html) or that approval for experimentation by that committee is needed for the strategy in question.

Related Strategies for Creating a Truly Comprehensive Approach

It is recommended that related strategies be included as candidates in any program planning process to create a truly comprehensive approach to the highway safety problems associated with this emphasis area. There are five types of related strategies:

• Public Information and Education (PI&E) Programs. Many highway safety programs can be effectively enhanced with a properly designed PI&E campaign. The primary goal with PI&E campaigns in highway safety is to reach an audience across an entire jurisdiction or a significant part of it. However, it may be desirable 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, where PI&E campaigns are deemed unique for the emphasis area, the strategy is explained in detail. As additional guides are completed for the AASHTO plan, they may 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 seatbelts. 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 the nature of how they must be performed, are conducted at specific locations. The effect (e.g., lower speeds, greater use of seatbelts, 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 seatbelt 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 it is desired to try an innovative and untried method, a pilot program is recommended. Within this guide, where the application of enforcement programs is deemed appropriate, the application is often in support of some other strategy. The other strategy may target either a whole system or a specific location. When enforcement programs are recommended for a strategy, the description for that strategy will suggest this possibility (see the attribute area for each strategy entitled “Associated Needs”). In some cases, where an enforcement program is deemed unique for the emphasis area, the strategy will be explained in detail. As additional guides are completed for the AASHTO plan, they may address the details regarding the design and implementation of enforcement strategies. For this particular emphasis area, speed enforcement may be particularly applicable. The reader is also directed to the FHWA report synthesizing speed research (http://www.fhwa.dot.gov/tfhrc/safety/pubs/speed/spdtoc.htm).
• Strategies to Improve Emergency Medical and Trauma System Services. Treatment of injured parties at highway crashes can have a significant impact on the level of severity and length of time that an individual spends in treatment. This is especially true with 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 that are included here are often thought of as simply support services, they can be critical to the success of a comprehensive highway safety program. Therefore, for this emphasis area, an effort should be made to determine if there are improvements that can be made to this aspect of the system, especially for programs that focus on location-specific (e.g., corridors) or area-specific (e.g., rural areas) issues. As additional guides are completed for the AASHTO plan, they may address the details regarding the design and implementation of emergency medical systems strategies.
• Strategies Directed at Improving the Safety Management System. The management of the highway safety system is foundational to success. There should be in place a sound organizational structure, as well as infrastructure of laws, policies, etc., to monitor, control, direct, and administer a comprehensive approach to highway safety. It is important that a comprehensive program not be limited to one jurisdiction, such as a state department of transportation (DOT). Local agencies often have the majority of the road system and its related safety problems to deal with. They also know, better than others, what the problems are. For example, the state of California created a task force composed of professionals from different fields to study the safety issues of some state highway corridors. The goal was to study the safety history of roadway segments with high-accident locations and to recommend improvements that could be implemented. This is an example of a program that could be employed on a larger scale to examine horizontal curve improvements. However, programs such as this require dedicated funding to be developed. As additional guides are completed for the AASHTO plan, they may address the details regarding the design and implementation of strategies for improving safety management systems. When that occurs, the appropriate links will be added from this emphasis area guide.
• Strategies that Are Detailed in Other Emphasis Area Guides. Crash statistics show that run-off-road (ROR) and head-on crashes are the most prevalent types of crashes at horizontal curves. Therefore, the strategies for improving safety at horizontal curves address ROR and head-on crashes. Implementation guides, similar to this one, have already been developed to address ROR and head-on crashes, and many of the strategies presented in this guide are common to strategies presented in the ROR and head-on guides (Volumes 6 and 4, respectively, of this report). For example, Strategy 15.2 A4 is to install shoulder rumble strips. Installation of shoulder rumble strips is also addressed in the ROR guide under Strategy 15.1 A1.

  When strategies for reducing curve-related crashes relate to strategies presented in the ROR or head-on guides, the strategies are presented in both guides for complete coverage of the topic. For these common strategies, this implementation guide on reducing curve-related crashes presents information on the expected effectiveness of the strategies. The reader is then referred to the ROR or head-on crash guides for additional information related to the strategies such as keys to success, potential difficulties, organizational, institutional, and policy issues. If particular issues that pertain specifically to horizontal curves are not covered in the ROR or head-on guide, these details are covered within the text of this guide. In this way, duplication of the topic is minimized, while complete coverage of the topic is still provided.

  When strategies for reducing curve-related crashes relate to strategies presented in detail in another guide, information on the strategy, specifically related to curves, is generally presented in the following manner:

  • General description of strategy,
  • Summary of effectiveness of treatments (presented in ROR or head-on guides),
  • Effectiveness of treatments (not discussed in ROR or head-on guides, or recent studies not included within ROR or head-on guides), and
  • Special issues pertaining to horizontal curves.

Objective 15.2 A: Reduce the Likelihood of a Vehicle Leaving its Lane and either Crossing the Roadway Centerline or Leaving the Roadway at a Horizontal Curve

Strategy 15.2 A1: Provide Advance Warning of Unexpected Changes in Horizontal Alignment (T)

General Description

The intent of this strategy is to provide advance warning to a driver that the horizontal alignment of the roadway is about to change and that the driver must alter the path,and possibly the speed, of the vehicle downstream of the warning to negotiate the curve safely. Advance warning of alignment changes should be provided to a driver when changes in alignment are unexpected. This typically occurs in situations where curves are sharper than anticipated or after a long tangent section of roadway.

Advance warning of alignment changes can be conveyed to the driver in numerous ways. The traditional approach is through the use of roadway signing. In the case of a “Curve” sign, the sign not only prepares the driver for a change in alignment, but it also provides information on whether the alignment turns to the left or to the right downstream of the sign. An advisory speed sign can be used to indicate a recommended speed through the curve. Flashing beacons can also be used with the “Curve” and advisory speed signs to draw more attention to these respective signs. Other methods of advance warning that have been used on a more limited basis include warning messages placed on the pavement and rumble strips in advance of the curve. These measures have been used primarily in advance of very sharp curves. In the case of rumble strips, the rumble strips are typically used in conjunction with “Curve” signs and advisory speed signs and are installed to call attention to the advisory speed signs. Note that installation of rumble strips in advance of curves may cause undesirable driving behaviors such as drivers purposely crossing over into the opposing lane to avoid the rumble strips. Also, some motorists (particularly truckers and motorcyclists) do not like the effects (vibration and sound) generated from the rumble strips. Other methods of advance warning involve pavement markings that try to cause a driver to reduce the speed of his/her vehicle through visual deception. These methods can include transverse lines with decreasing spacing or edgelines that give the appearance of a narrowing lane width. Research is underway in National Cooperative Highway Research Program (NCHRP) Project 3-61 to develop a methodology by which horizontal curve information can be conveyed to motorists in a more consistent and reliable fashion.

This strategy focuses on providing drivers with advance warning of the horizontal curve. In some cases it is sufficient just to heighten the awareness of the driver that he/she is approaching a change in alignment. In other situations, advance warning treatments try to influence the speed of the driver on the approach to the curve. Affecting speeds on the tangent sections preceding horizontal curves is particularly important because excessive speed is a significant factor in crashes at horizontal curves. Moreover, research has shown that drivers do not fully adjust their speeds on the approach. The speed at which a vehicle enters a curve relates more to the speed of the vehicle as it approaches the curve (which is based in part on the driver’s response to the preceding alignment) than to the sharpness of the curve (Retting and Farmer, 1998).

This strategy closely relates to Strategy 15.1 A4 in the guide for addressing ROR accidents (Volume 6 of this report), which pertains to enhanced delineation of sharp curves for reducing ROR crashes. Strategy 15.1 A4 focuses on innovative and experimental onpavement markings (nontraditional treatments) that provide advance warning of horizontal curves. Strategy 15.2 A1 pertains to both traditional and nontraditional advance warning treatments at horizontal curves. These treatments do not provide the driver with a view of the curve. On the other hand, Strategy 15.2 A2 focuses on delineation treatments installed along the curve, which provide the driver with a picture of the sharpness of the curve.

Summary of Effectiveness of Nontraditional Treatments at Horizontal Curves

This section provides a brief summary of what is known about the safety effectiveness of nontraditional treatments that provide advance warning to horizontal curves, as presented in the ROR guide. Several variations of nontraditional pavement marking treatments have been experimented with to improve safety at horizontal curves. The two most promising treatments are the pavement arrow (Exhibit V-2) and transverse striping treatments. In general, these nontraditional pavement marking treatments have reduced both speeds and accidents at horizontal curves in experiments conducted by a few agencies. However, it has yet to be determined how effective these nontraditional pavement marking treatments will be when installed on a broader basis and over the long term at a given site.

Exhibit V-2 Pavement Arrow (Photo Provided by PennDOT)

Effectiveness of Traditional Advance Warning Treatments at Horizontal Curves

The Manual on Uniform Traffic Control Devices (MUTCD) (USDOT, 2003) indicates that horizontal alignment signs (Turn [W1-1], Curve [W1-2], Reverse Turn [W1-3], Reverse Curve [W1-4], or Winding Road [W1-5]) may be used in advance of situations where the horizontal roadway alignment changes. The One-Direction Large Arrow (W1-6) sign may be used on the outside of the turn or curve. If the change in horizontal alignment is 135 degrees or more, the Hairpin Curve (W1-11) sign may be used. If the change in horizontal alignment is approximately 270 degrees, such as on a cloverleaf ramp, the 270-degree Loop (W1-15) sign may be used. Additional warning also may be provided by use of the “Advisory Speed” plaque (W13-1) that is intended to indicate the maximum recommended speed around a curve. The MUTCD states that the Advisory Speed plaque shall be used where an engineering study indicates a need to advise road users of the advisory speed for a condition.

Research suggests that the proliferation of curve warning signs, especially those supplemented with advisory speed plates, may have lessened the average motorist’s respect for the messages that they convey (Lyles, 1980). However, because of tort liability concerns, many highway agencies prefer to use traditional advance warning and curve signs even if research indicates that these signs may be ineffective. The findings from studies that investigated the effectiveness of traditional advance warning signs are summarized in the following paragraphs.

Lyles (1980) examined the effectiveness of five sign treatments for controlling driver speeds in the vicinity of hazardous horizontal curves on rural two-lane highways. Sign treatments ranged from the standard curve warning sign to a regulatory speed zone sign in conjunction with a curve warning sign. The effectiveness of the signs was evaluated based on speeds of motorists as they approached and negotiated the horizontal curves and whether vehicles crossed over center and edgeline markings. Lyles found that no sign, or group of signs, was consistently more effective than another in decreasing the potential hazard at horizontal curves.

Zwahlen (1983) examined the effectiveness of advisory speed plates in causing drivers to reduce their speeds through curves. He concluded that advisory speed signs are not more effective in causing drivers to reduce their speeds through curves than the curve signs alone are, at least not in dry weather, and that further research was needed to determine the effectiveness of advisory speed signs in adverse weather conditions. Zwahlen recommended that advisory speed sign maintenance, especially new installations, be given a low priority.

Ritchie (1972) examined the choice of speed in driving through curves as a function of advisory speed and curve signs. He found that motorists drove faster and produced more lateral acceleration when (a) a curve sign was present, and (b) an advisory speed sign was present, than under the opposite conditions. In addition, motorists exceeded advisory speed signs of 24 to 56 km/h (15 to 35 mph), but motorists did not exceed advisory speed signs of 72 to 80 km/h (45 to 50 mph). Ritchie concluded that advance warning signs serve to reduce uncertainty and allow drivers to proceed with greater confidence.

One of the reasons for the low percentage of compliance with posted advisory speeds on curves may be that the criteria for setting advisory speeds on curves are outdated due to advances in vehicle characteristics. The current criteria for setting advisory speeds on curves have remained essentially unchanged for more than 50 years. Chowdhury et al. (1998) evaluated the validity of current criteria for determining advisory speeds on horizontal curves and concluded that the criteria are not valid for modern vehicles. At most curves, posted advisory speeds were well below the prevailing traffic speed and below the recommended values suggested by the two methods for determining advisory speeds, namely the ball-bank indicator and the Traffic Control Devices Handbook (TCDH) (Institute of Transportation Engineers, 2001).

While the previously mentioned studies suggest that traditional advance warning treatments are not effective in decreasing the potential hazard at horizontal curves, several studies suggest otherwise. Hammer (1968) evaluated the effectiveness of various types of minor improvements in reducing accidents. Two of the minor improvements included in the evaluation were the installation of curve warning signs and advisory speed signs at horizontal curves. Hammer found that curve warning signs reduced accidents by 18 percent at horizontal curves and that installation of both curve warning and advisory speed signs reduced accidents by 22 percent. Leisch (1971) also reported advisory speed signs to be effective in reducing accidents at horizontal curves.

Hanscom (1976) evaluated a slightly different scenario. He evaluated the effects of signing to warn drivers of wet weather skidding hazards at horizontal curves. Three curved highway sections were treated using five experimental sign treatments. The primary measure of effectiveness was mean speed at the critical curve locations. In particular, the target sample was the highest quartile speed group of vehicles arriving in advance of the curve. Significant speed reductions were observed at critical curve locations during conditions of wet pavements when warning signs were supplemented with flashing beacons. Therefore, Hanscom recommended that activated warning signs be used at critical curve locations as a skidding accident countermeasure.

Several other types of traditional advance warning treatments that have not necessarily been evaluated for their safety effectiveness at horizontal curves include oversized warning signs and double-posted signs. The MUTCD (USDOT, 2003) indicates that oversized warning signs may be used where speed, volume, and other factors result in conditions where greater visibility or emphasis would be desired, such as at unexpected or sharp horizontal curves. Agencies have also double-posted warning signs to draw greater attention to warning signs.

In summary, none of the studies designed to evaluate the effectiveness of traditional advance warning treatments at horizontal curves question the importance of providing a curve warning sign in advance of unexpected or sharp curves, but conflicting results have been obtained on the effectiveness of advisory speed signs. The most recent studies suggest that advisory speed signs do not garner respect from the average motorist. These studies conclude that advisory speed signs do not effectively reduce speeds at horizontal curves.

Before drawing conclusions regarding the effectiveness of advisory speed signs on improving safety at horizontal curves, two issues should be considered. First, of the studies cited above, only Hammer evaluated the effectiveness of advisory speed signs using accident data. The other studies used speed as the measure for evaluating the effectiveness for advisory speed signs. Second, Hanscom is the only reference cited above that recommends targeting the highest quartile speed group of vehicles when evaluating the effectiveness of advance warning treatments based upon speed. He suggests that these vehicles are the vehicles most likely to be involved in accidents at horizontal curves.

Strategy Attributes

The ROR guide presents attributes under Strategy 15.1 A4 that are common to this strategy. The reader is, therefore, directed to that guide for more detailed information related to this strategy. However, three additional points should be considered in addition to what is presented within the ROR crash guide.

First, for higher compliance with posted advisory speed signs, a new set of criteria should be developed for setting advisory speeds. Chowdhury et al. (1998) recommended determining the advisory speed based on a sample of vehicle speeds, but other alternatives should be investigated as well. If a new set of criteria is developed for setting advisory speed, curves currently posted with advisory speed signs will have to be re-evaluated and new advisory speeds will have to be posted, and in some cases the advisory speed signs may be removed completely. A public information effort may be needed to re-educate the driving public until drivers once again respect this type of advisory sign.

Second, an important key to success is identifying sites where treatments of this nature have the potential to improve safety. This strategy targets curves where changes in alignment are unexpected and drivers may need to reduce their speeds to negotiate the curve safely. Third, in several of the studies previously mentioned, speed, or more specifically change in speed, has been used as a surrogate measure for evaluating the effectiveness of a treatment.

Such a relationship should be established by further research. If speed is studied as a surrogate measure, consideration should be given to Hanscom’s recommendation of targeting the highest quartile speed group of vehicles when evaluating the effectiveness of both traditional and nontraditional advance warning treatments.

Strategy 15.2 A2: Enhance Delineation Along the Curve (T)

General Description

This strategy focuses on providing the driver with better visual cues to recognize the presence and geometry of the curve. Various methods are available to provide delineation along a curve. Some traditional delineation devices such as chevrons, post-mounted delineators, and delineators placed on guardrail are located outside the roadway shoulder along the curve, while others, such as lane lines or edgelines and raised pavement markers, are placed on the surface of the traveled way. Several nontraditional devices, such as light-emitting diode (LED) in-pavement luminaires and LED barrier-mounted guidance tubes, have also been used for delineation purposes.

Agencies generally implement three levels of delineation based on the context of the location:

• For tangents,
• For most curves, and
• For problem curves (e.g., high-accident locations).

This strategy primarily addresses delineation along problem curves, but it may also be applied to most curves. Enhanced delineation of a curve serves two purposes. First, it can provide a better view of the curve on the approach tangent. The degree to which this works well depends in part upon a combination of road factors, including horizontal and vertical alignments, obstructions on the inside of the curve, and the types of delineation devices used. Delineation helps prepare the driver for the approaching change in horizontal alignment. Roadside delineators are particularly effective in providing this advanced view of the curve. In many cases, delineation devices increase the preview sight distance on the approach tangent.

Second, as the driver traverses the curve, the delineation device provides a continuous feature for positive guidance. This helps the driver position his/her vehicle within the proper travel lane while negotiating the curve.

This strategy is related to Strategy 15.1 A6 in the ROR guide, pertaining to better pavement markings at appropriate locations. Strategy 15.1 A6 in the ROR crash guide does not provide detailed information on post-mounted delineators or chevrons. Therefore, a more detailed discussion of these types of roadside delineation is provided below.

Summary of Effectiveness of Better Pavement Markings

This section provides a brief summary of the safety effectiveness of better pavement markings, as presented in the ROR guide. “Better pavement markings” are pavement markings that are more durable, are all-weather, or have a higher retroreflectivity than traditional pavement markings. Raised pavement markings and wider edgelines are two approaches to enhancing delineation at a curve. These treatments are designed to help drivers who might leave the roadway because of inability to see the edge of the pavement along the horizontal curve.

The actual safety benefits of such treatments are difficult to assess. Raised pavement markers provide for increased delineation of the driving path and enhance the ability of the driver to track the roadway, particularly under nighttime, wet-weather, or adverse-weather conditions. Raised pavement markers also can provide tactile and auditory warnings to drivers, similar to rumble strips, when vehicles traverse the markers. When used at isolated curves rather than continuously along the alignment, raised pavement markers may provide greater emphasis to the change in roadway alignment. Several studies have noted significant reductions in accidents because of the installation of raised pavement markers. Despite the noted advantages of raised pavement markers and the positive research, some studies have indicated an increase in nighttime accidents after the installation of raised pavement markers. Therefore, the safety effectiveness of raised pavement markers is questionable. Concerning wider edgelines such as 20-mm (8-in.) edgelines versus 10-mm (4-in.) edgelines, the effectiveness of raised pavement markers in reducing ROR crashes has not been satisfactorily demonstrated in the research literature, although the New York DOT indicates that wider edgelines have the potential to reduce ROR crashes on two-lane roads by 10 to 15 percent.

Post-Mounted Delineators and Chevrons

Post-mounted delineators and chevrons are two types of delineation treatments that are installed outside of the roadway. They are intended to warn drivers of an approaching curve and provide tracking information and guidance to the drivers. While they are intended to act as a warning, it should also be remembered that the posts, placed along the roadside, represent a possible object with which an errant vehicle can crash. Design of posts to minimize damage and injury is an important part of the considerations to be made when selecting these treatments.

In NCHRP Report 440, Fitzpatrick et al. (2000a) report the results of several studies on postmounted delineators. They report that post-mounted delineators reduce the accident rate only on relatively sharp curves during periods of darkness. In addition, highways with postmounted delineators have lower accident rates than highways without post-mounted delineators, and the cost of post-mounted delineators are justified for highways with average daily traffic (ADT) exceeding 1,000 vehicles per day. Fitzpatrick et al. do not quantify the effectiveness of post-mounted delineators in reducing curve-related crashes. Bali et al. (1978) provide similar results.

Krammes and Tyer (1991) evaluated the operational effectiveness of raised pavement markers as an alternative to post-mounted delineators at horizontal curves on two-lane rural highways. They evaluated nighttime speed and lateral placement data from five sites. For both short-term and intermediate-term analyses, vehicle operations with raised pavement markers compared favorably with operations when post-mounted delineators were present. Vehicle operations were not significantly affected on the inside lane of the curve, but significant differences were observed on the outside lane of the curve. Speeds at the midpoint of the curve were consistently 1.6 to 4.8 km/h (1 to 3 mph) higher with the raised pavement markers, and the mean lateral placement of vehicles was consistently 0.3 to 0.6 m (1 to 2 ft) further from the centerline at the midpoint of the curve with the raised pavement markers than with the post-mounted delineators. In addition, the variability in lateral placement of vehicles at the midpoint of the curve was less with raised pavement markers than with post-mounted delineators.

Zador et al. (1987) examined the short- and long-term effects of chevrons, post-mounted delineators, and raised pavement markers on the speed and placement of vehicles traveling on curves on rural two-lane highways. In general, all three delineation treatments affected driver behavior at night. Vehicle paths were shifted away from the centerline on horizontal curves where raised pavement markers and chevrons were installed and toward the centerline on curves where post-mounted delineators were used. Vehicle speed and placement variability were also slightly reduced with the use of chevrons and raised pavement markers. Zador et al. did not conclude that one delineation treatment was superior to the others and indicated that the primary benefit of any of these delineation treatments may simply be that they help drivers better recognize that they are approaching a curve.

Agent and Creasey (1986) investigated the ability of various traffic control devices to delineate horizontal curves so drivers would perceive the curve and slow to an appropriate speed and so drivers would have improved guidance through the curve. The investigation consisted of both laboratory tests and field data collection. The laboratory tests suggested that increasing the height of the post-mounted delineator while maintaining the distance from the post to the pavement edge, and keeping the post spacing constant, made a curve appear sharper than other delineator devices. From speed data, encroachment data, and some accident data, Agent and Creasey found that pavement markings had a greater effect on drivers than post-mounted delineators installed on the roadside did. In addition, chevrons had slightly more influence on speeds and encroachments than other post-mounted delineators did.

Jennings and Demetsky (1985) evaluated the effectiveness of three post-mounted delineator systems in controlling ROR crashes. The post-mounted delineator systems were evaluated based upon changes in speed and lateral placement of vehicles within the travel lane. Jennings and Demetsky found that drivers reacted most favorably to chevron signs on sharp curves greater than or equal to 7 degrees (radius of 250 m [820 ft]) and to standard postmounted delineators on curves less than 7 degrees.

In summary, the safety effectiveness of enhanced delineation at a horizontal curve is difficult to assess because many of the research results are conflicting. Part of the difficulty arises because several of the studies use modifications in speed and lateral placement as surrogate measures to evaluate safety rather than actual crash data. The general conclusions that may be drawn regarding the safety effectiveness of enhanced delineation at horizontal curves are that post-mounted delineators may improve safety at sharp curves and that chevrons are more effective than standard post-mounted delineators are. At this point, no quantitative estimates of the safety effectiveness of enhanced delineation treatments can be made. Zador et al. may have summarized the safety effectiveness of enhanced delineation best by indicating that the primary benefit may simply be that enhanced delineation treatments help drivers better recognize that they are approaching a horizontal curve.

Strategy Attributes

The ROR guide presents attributes common to Strategies 15.2 A2 and 15.1 A6. The reader is directed to the ROR guide for more detailed information related to this strategy.

Strategy 15.2 A3: Provide Adequate Sight Distance (T)

General Description

Sight distance is a fundamental element in geometric design. The amount of sight distance provided to the driver is a function of the three-dimensional features of the highway—the cross section (roadside), vertical alignment (grades and vertical curves), and horizontal alignment. At horizontal curves, obstructions that limit the driver’s sight distance come in many shapes and forms. The road surface may be the sight obstruction if the horizontal curve is located on a crest vertical curve. Physical features outside of the traveled way—such as trees or bushes, guardrail or concrete barriers, and the backslope of a cut section—also can limit the driver’s sight distance. As trees and other roadside vegetation mature, the sight distance at a horizontal curve may change. Motor vehicles and other road users can also create temporary sight obstructions. Efforts should be made to ensure that obstructions do not reduce the sight distance at a horizontal curve to less than the minimum stopping sight distance.

The available stopping sight distance on a roadway should be sufficiently long to enable a vehicle traveling at or near the design speed to stop before reaching a stationary object in its path (AASHTO, 2001). Providing at least the minimum stopping sight distance at every point along a roadway is critical for safe operations.

Stopping sight distance is the sum of (1) the distance traversed by the vehicle from the instant the driver sees an object necessitating a stop to the instant the brakes are applied (i.e., the brake reaction distance) and (2) the distance needed to stop the vehicle from the instant the brakes are applied (i.e., the braking distance). Furthermore, drivers have other sight distance needs in addition to stopping for hazards in the paths of their vehicles. A driver needs an adequate view of the roadway alignment and roadway features ahead for safe control and guidance of the vehicle (Gattis and Duncan, 1995). This sight distance to the roadway surface and other appurtenances ahead is referred to as preview sight distance (PVSD). A roadway designed with geometric design features adequate to the design speed would in many cases provide sufficient PVSD; however, a roadway with constrained design features, or a roadway section that does not conform to current geometric design policies, could have inadequate PVSD.

Current design policy does not identify where lack of sight distance may produce a significant safety risk. NCHRP Report 400 (Fambro et al., 1997) suggests that most locations with limited stopping sight distance experience very few accidents. However, limited stopping sight distance may be a greater concern where an intersection or driveway is present along a horizontal curve. Objective 17.1 C in the unsignalized intersection guide provides more detailed information about intersection sight distance.

If the available sight distance is found to be less than the minimum stopping sight distance, the sight obstruction should be removed or the roadway should be realigned to provide at least minimum stopping sight distance. The safety effectiveness of increasing sight distance will be a function of the amount of the sight restriction, the traffic volume exposed to it, and the presence of other conditions contributing to risk. For additional information on stopping sight distance, see NCHRP Report 400 (Fambro et al., 1997).

Exhibit V-3
Strategy Attributes for Providing Adequate Sight Distance (T)

Technical Attributes
Target	Drivers of vehicles approaching a curve with limited sight distance.
Expected Effectiveness	It is difficult to determine the expected safety benefits of improving the sight distance at a horizontal curve when the available sight distance is slightly less than the minimum stopping sight distance. The accident statistics do not provide a sufficient amount of information to determine the expected safety benefits. There is some indication from research (see NCHRP Report 400) that improving locations with substantial sight distance restrictions offers safety benefits.
Keys to Success	Seasonal changes and growth of roadside vegetation can alter the available sight distance at horizontal curves. Therefore, one of the keys to success is developing a program to periodically check the offset to roadside vegetation along horizontal curves. Another key to success is to institute a policy that requires checking horizontal sight distance when any installation is made along a curve, such as new guardrail or attenuation devices, as well as barriers used during construction and maintenance activities.
Potential Difficulties	There is a lack of data to show the effect of inadequate sight distance on safety. Thus, at horizontal curves where the available sight distance is less than the minimum stopping sight distance, it might be difficult to justify spending funds to improve sight distance, unless there is a documented history of crashes due to inadequate sight distance. This is particularly true when the cost to improve the sight distance could be extremely high, such as at a bridge pier, a backslope of a cut section, or the roadway surface.
Appropriate Measures and Data	Process measures of program effectiveness would include the number of horizontal curves where the sight distance was improved and the change in minimum sight distance. Impact measures include the number of total crashes reduced at these locations and changes in total crashes. Surrogate measures, such as lane position, or lane departures, may be appropriate for short-term evaluation. Accurate crash and exposure data are needed for before and after periods on treated sections and similar comparison groups to evaluate the effectiveness of this treatment. Geometric data on the curve (i.e., length and degree of horizontal curvature, as well as vertical geometry along the curve) and available sight distance are needed also.
Associated Needs	Available sight distances can be determined from roadway plans. However, a combination of the use of video logs and site visits may be required to identify actual sight obstructions and to measure available sight distances in the field.
Organizational and Institutional Attributes
Organizational, Institutional and Policy Issues	None identified.
Issues Affecting Implementation Time	The type of sight obstruction will affect the implementation time. When the sight obstruction is roadside vegetation (trees, shrubs, etc.), routine trimming can be scheduled to eliminate the obstruction so that the implementation time will be short. However, when the sight obstruction is a bridge pier, a backslope of a cut section, or the roadway surface, eliminating the obstruction may require realignment of the roadway or some major earth work that could require an environmental review. In this case, not only would the construction time be longer, but an environmental review would also extend the implementation time.
Costs Involved	The type of sight obstruction is the major determinant of cost. The costs may only include maintenance costs. However, they could include costs for replacement of installations or realignment of the curve, in which case design, construction, and maintenance costs will be involved.
Training and Other Personnel Needs	No special needs identified.
Legislative Needs	None identified.
Other Key Attributes
	None identified.

Strategy 15.2 A4: Install Shoulder Rumble Strips (P)

General Description

This strategy focuses on issues related to the safety effectiveness of shoulder rumble strips at horizontal curves (Exhibit V-4). While shoulder rumble strips are designed primarily to reduce ROR crashes, they can also reduce head-on crashes. Head-on crashes may occur when a vehicle leaves the roadway and its driver overcompensates while trying to recover control of the vehicle, sending the vehicle into the opposing traffic lane.

EXHIBIT V-4 Shoulder Rumble Strips (http://safety.fhwa.dot.gov/programs/rumble.htm)

One of the unique issues related to the safety effectiveness of shoulder rumble strips at horizontal curves concerns the departure angle for vehicles that leave the roadway. Studies indicate that an average departure angle for ROR crashes ranges between 3 and 8 degrees (Hall, 1991; O’Hanlon and Kelley, 1974). In these studies, it is not clear whether the analyses included crashes that occurred along tangent sections of highway, crashes that occurred at horizontal curves, or both. Regardless, the research results indicate that ROR crashes typically occur at shallow departure angles. However, the departure angle is a function of the horizontal alignment. If a vehicle drifts to the right along a tangent section of roadway at an angle of 3 degrees, the vehicle follows a certain path onto the roadside. If the same vehicle path occurs along a horizontal curve, the result will be a larger departure angle because of the curvature of the roadway. This has two implications on the effectiveness of shoulder rumble strips at horizontal curves. First, because the vehicle crosses the rumble strips at a greater angle, an inattentive driver has less exposure time to the stimuli (i.e., vibration and noise) generated by the rumble strips. Second, if the shoulder width on the curve is the same as on an adjacent tangent, the driver has less time to regain control of the vehicle before it leaves the shoulder.

This strategy is the same as Strategy 15.1 A1 provided in the ROR guide. The section below provides a summary of the effectiveness of shoulder rumble strips. Following that is a section that presents special issues concerning shoulder rumble strips at horizontal curves.

Summary of Safety Effectiveness of Shoulder Rumble Strips

Shoulder rumble strips have proven to be an effective measure in reducing the number of ROR crashes on freeways. Numerous studies have quantified the reductions in ROR crashes to varying degrees. In general, the studies indicate that ROR crashes were reduced by 20 to 50 percent because of the installation of shoulder rumble strips. The section of this guide on the description of the problem indicates that horizontal curve fatal crashes primarily occur on rural two-lane highways. Because little is known about the effectiveness of shoulder rumble strips on two-lane highways, the ROR guide suggests that one might assume a similar experience to what has been documented on rural freeways—a 20- to 30-percent reduction in single-vehicle ROR crashes. However, the reader should review the considerations that are listed below before making such an estimate.

Strategy Attributes

The ROR guide presents attributes common to this strategy under Strategy 15.1 A1. The reader is directed to the ROR guide for more detailed information. In addition, however, several issues should be pointed out, particularly regarding the installation of shoulder rumble strips at horizontal curves.

Studies concerning the safety effectiveness of shoulder rumble strips have used crash data collected over long segments of highway, meaning that the study segments included both tangents and horizontal curves. No distinction was made in these studies between tangent and horizontal curve sections, and there are no studies that analyze the effectiveness of shoulder rumble strips at horizontal curves only. It might thus be assumed that similar safety benefits apply to the application of shoulder rumble strips along both types of alignments (tangents and curves). However, it should be recognized that the effectiveness of shoulder rumble strips in reducing ROR crashes depends on various elements, including the frequency with which vehicles in the traffic stream run off the road, the vehicle departure angle, the vehicle speed, the shoulder width, and the roadside environment. The vehicle departure angle is of particular interest in this case.

It is, therefore, a complex issue to speculate on the specific effectiveness of shoulder rumble strips specifically on curves. Consider some of the following attributes:

1. The proportion of vehicles that run off the road is expected to be significantly greater on a curve than on a tangent section of road.
2. Hall (1991) and Elefteriadou et al. (2001) have conducted research on shoulder rumble strips and vehicle departure angle. Vehicle departure angle is a function of the steering angle and the curvature of the roadway. As the vehicle departure angle increases, the exposure time to stimuli generated by shoulder rumble strips and the available recovery distance decreases, making it less likely that the errant vehicle can recover in the available time.
3. Shoulder rumble strips installed along horizontal curves can also serve as an effective means of locating the edge of the travel way during inclement weather (FHWA, 2001). When drivers have difficulty seeing the edgeline along a horizontal curve (such as under heavy rain, light snow, or foggy conditions), a shoulder rumble strip can help drivers maintain their proper lane position.
4. The potential difficulties most often associated with shoulder rumble strips include snow removal (i.e., potential damage to snow plows and rumble strips), drainage, shoulder maintenance, noise, motorcycle use, and bicycle use. Each of these potential difficulties is discussed in the ROR guide. Snow removal, drainage, maintenance, noise, and motorcycle use are often listed as potential difficulties, but experience has shown that these concerns can often be dealt with or dismissed through sensible policies and targeted application of the solution.

Incompatibility between shoulder rumble strips and bicycle use is a concern in some locales. For example, a moratorium on the installation of ground-in rumble strips where bicycles were allowed was initiated in California until further research on the subject is completed (Bucko and Khorashadi, 2001). The three most comprehensive studies on the effects that rumble strips have on bicyclists were conducted in Pennsylvania, California, and Colorado by Elefteriadou et al. (2000), Bucko and Khorashadi, and Outcalt (2001a), respectively. Each study included bicycle and motor vehicle testing of various rumble strip designs. In general, the rumble strips that provided the greatest amount of stimuli (i.e., noise and vibration) to alert an inattentive or drowsy driver also were the most uncomfortable for the bicyclists to traverse. Likewise, the rumble strips that were the most comfortable for the bicyclists generated the least amount of stimuli in a motor vehicle to alert a drowsy or otherwise inattentive driver. In all three studies, compromises were made when selecting the rumble strip design most compatible for both types of road users.

Shoulder width is a major issue to consider before installing shoulder rumble strips. For further details on designs used by some states, see Appendix 1.

Finally, the impact of rumble strips on pavement performance is an issue that is often overlooked. Because rumble strips reduce the effective structural cross section of the pavement, rumble strips may reduce the overall pavement life or require greater total pavement thicknesses if significant loadings are anticipated to the shoulder or rumble strip area. Elefteriadou et al. (2001) provide some discussion of pavement integrity issues related to rumble strip installation.

For additional information on shoulder rumble strips, Synthesis of Shoulder Rumble Strip Practices and Policies was recently published by SAIC (2001), and FHWA published Technical Advisory for Roadway Shoulder Rumble Strips (FHWA, 2001). Both documents are available from the FHWA rumble strip Web site (http://safety.fhwa.dot.gov/programs/rumble.htm).

It should be noted that the ROR guide addresses the use of mid-lane rumble strips (Strategy 15.1 A3), which serve a similar purpose to shoulder rumble strips except that mid-lane rumble strips are installed in the center of the travel lane instead of on the shoulder.

Strategy 15.2 A5: Install Centerline Rumble Strips (T)

General Description

Centerline rumble strips are installed primarily to reduce head-on and sideswipe crashes along undivided roadways. Their primary function is to alert drowsy or otherwise inattentive drivers that their vehicles are encroaching upon the opposing lane through tactile and auditory stimulation. Centerline rumble strips may also discourage drivers from cutting across the inside of a curve. There is no standard design for centerline rumble strips, but generally the rumble strips are either (1) located along the width of the centerline pavement markings (Exhibit V-5), extending into the travel lane by as much as 0.5 m (1.5 ft) (Exhibit V-6), or (2) placed on either side of the centerline (Exhibit V-7). Some states install rumble strips continuously along the centerline, while other states install centerline rumble using a skip pattern.

Installing centerline rumble strips directly relates to Strategy 18.1 A1 in the guide for addressing head-on collisions, which is centerline rumble strips for two-lane roads. Centerline rumble strips are a relatively new strategy for reducing head-on crashes. Subsequently, little information is available on the safety effectiveness of this type of rumble strip. The section immediately following this one summarizes the effectiveness of centerline rumble strips as presented in the head-on guide, as well as the results of three recent studies on centerline rumble strips. The final section under this strategy discusses strategy attributes that relate to applying centerline rumble strips at horizontal curves.

EXHIBIT V-5 Centerline Rumble Strips (Photo Provided by PennDOT)

EXHIBIT V-6 Centerline Rumble Strips (Photo Provided by Caltrans)

EXHIBIT V-7 Centerline Rumble Strips (Photo Provided by MnDOT)

Summary of Safety Effectiveness of Centerline Rumble Strips

The head-on guide identifies two studies that showed centerline rumble strips to be effective in reducing head-on crashes. Centerline rumble strips were installed on a two-lane, undivided rural highway in Delaware (Perrillo, 1998). During the 36-month before period, there were 6 fatal crashes, 14 injury crashes, and 19 property damage only crashes. During the 24-month after period, there were 0 fatal crashes, 12 injury crashes, and 6 property damage only crashes. It was concluded that the centerline rumble strips reduced the total number of crashes and the severity of the crashes. In California, improvements were made to a 32-km (20-mi) segment of a rural two-lane highway to reduce the number of head-on crashes (Fitzpatrick et al., 2000a). The improvements included replacing the double yellow stripes with centerline rumble strips and raised profile thermoplastic traffic striping. In addition, raised pavement markers were installed between the rumble strips and raised profile thermoplastic. Using 34 months of before data and 25 months of after data, an evaluation showed that the centerline rumble strips and other improvements reduced the crash frequency from an average of 4.5 crashes per month in the before period to 1.9 crashes per month in the after period.

Rys et al. (2003) conducted a study to determine the most effective centerline rumble strip pattern for use on Kansas roadways. A survey of the few agencies that currently use centerline rumble strips found that there was no generally accepted pattern as to the types and dimensions of these rumble strips. Using information gathered on some of the patterns currently used, a test section of roadway was prepared by installing 12 different sections of centerline rumble strips of varying dimensions and spacing. Seven test vehicles were chosen to represent an accurate range of roadway traffic, and two measurements were taken in each automobile: interior noise level and steering wheel vibration level. Based on their findings, Rys et al. recommended that further testing be conducted on two of the centerline rumble strip patterns to more clearly determine which is most beneficial: either the continuous pattern that is 305 mm (12 in.) on the center and 305 mm (12 in.) long or the alternating pattern that is 305 mm and 610 mm (12 in. and 24 in.) on the center and 305 mm (12 in.) long. The authors noted that each of these patterns rated high in both noise level and vibration created; both were installed along urban and rural roadways in Kansas for continued testing during the 2003 year.

Mahoney et al. (2003) conducted a before-after study to determine whether centerline rumble strips have an effect on the lateral displacement of vehicles. Data were collected on eight roadway sections (four test and four comparative sections) in rural settings so that an operational analysis, rather than a safety analysis, could be performed on centerline rumble strips. To reduce the effect of outside influences on the lateral placement, tangent segments were chosen with minimal grade, no roadside barriers, and nominal horizontal curvature. The data analysis revealed that the mean lateral placement of vehicles shifted 140 mm (5.5 in.) away from the center of the lane subsequent to centerline rumble strip installation along roadway sections with 3.6-m (12-ft) lanes, and the mean lateral placement of vehicles shifted 76 mm (3 in.) away from the center of the lane subsequent to centerline rumble strip installation along roadway sections with 3.3-m (11-ft) lanes. Introduction of centerline rumble strips also decreased the amount of lateral placement variance that, in previous studies, had been shown to possibly increase traffic safety. The effects of centerline rumble strips on vehicle speed were inconclusive.

In 2001, Colorado DOT completed a before-after evaluation of 27 km (17 mi) of centerline rumble strips installed along a winding, two-lane mountain road (Outcalt, 2001b). The analysis used 44 months of before data and 44 months of after data. The resulting crash data and associated percent changes are shown in Exhibit V-8.

Exhibit V-8
Colorado Before-After Crash Summary of Centerline Rumble Strips (Outcalt, 2001b)

	Before Period 7/1/92–3/1/96 (44 months)	After Period 7/1/96–3/1/00 (44 months)	Percent Change
head-on crashes	18	24
head-on crashes per million vehicles	2.91	1.92	-34%
Sideswipe opposite direction crashes	24	18
Sideswipe opposite direction crashes per million vehicles	3.88	2.46	-36.5%
Average ADT	4628	5463	+18%

Strategy Attributes

The head-on guide presents attributes common to Strategy 15.2 A5 and Strategy 18.1 A1. The reader is directed to the guide for addressing head-on collisions for more detailed information related to this strategy. In addition, several issues should be highlighted concerning the effectiveness of centerline rumble strips, policy issues, and potential difficulties.

When considering the expected effectiveness of centerline rumble strips in reducing head-on crashes at horizontal curves, similar issues to those discussed in Strategy 15.2 A4 should be considered.

Arizona, California, Colorado, Delaware, Kansas, Maryland, Massachusetts, Minnesota, Oregon, Pennsylvania, Virginia, Washington, and Wyoming are among the states that have installed centerline rumble strips. To learn more about Minnesota DOT’s experiences with centerline rumble strips see http://www.dot.state.mn.us/d3/newsrels/03/10/06_rumble_strips.html. These installations have primarily been on an experimental basis. After agencies have sufficient experience with this new technique, a written policy should result for centerline rumble strips. The policies may include guidelines or recommendations regarding the type of sites at which to install centerline rumble strips, as well as design specifications and pavement thickness requirements.

The possibility of centerline rumble strips adversely affecting motorcyclists and inhibiting passing maneuvers is mentioned in the guide for addressing head-on collisions. However, experiences in Pennsylvania, Washington, and Minnesota suggest that this may be more a perceived problem than an actual problem. In Connecticut, however, centerline rumble strips were installed on a short section (less than 1.6 km [1 mi]) of a state route that carried a high percentage of truck traffic. The centerline rumble strips were removed after approximately 8 months because of complaints about noise.

Other potential disadvantages of centerline rumble strips include decreased visibility of centerline pavement markings, potential drainage problems, and snow removal difficulties. However, experience has not proven these potential disadvantages to be significant or insurmountable. In fact, the opposite may be true in some cases. At least one agency with centerline rumble strip installations has noted that the visibility of centerline pavement markings is not diminished because of centerline rumble strips and that centerline pavement markings are even visible when the rumble strips are filled with water. Likewise, at horizontal curves where greater superelevation can be expected, interruption of drainage flow patterns should be minimal. Concerning snow removal difficulties, no agency has indicated a reduction in pavement life because of centerline rumble strips. But centerline rumble strips have only recently been implemented, so more time is necessary to adequately address this issue. However, at least one agency has received comments that motorists perceived the centerline rumble strips as beneficial during snowy conditions because the motorists were still able to hear and feel the rumble strips in that kind of weather. An added benefit of centerline rumble strips is that they may extend centerline marking life because they decrease the number of vehicles crossing the markings.

Finally, the primary purpose of centerline rumble strips is to reduce head-on and sideswipe crashes. Centerline rumble strips also have the potential to reduce ROR crashes that occur to the left. If vehicles traveling on the inside of a curve cross the centerline, the centerline rumble strips alert the driver as soon as the vehicle encroaches on the centerline. This maximizes the recovery time and distance for vehicles that can run off the road to the left.

Strategy 15.2 A6: Prevent Edge Dropoffs (T)

General Description

Preventing edge dropoffs, also referred to as edgedrops, can reduce both ROR and head-on crashes by enabling a driver to recover an errant vehicle in a more controlled fashion. Edge dropoffs are a significant difference in elevation between the edge of traveled way and shoulder (Exhibit V-9). Edge dropoffs may occur after resurfacing or as the result of weather or vehicle-related settlement and can occur whether the shoulder is paved or not. Edge dropoffs may be more common on curves than on tangents.

EXHIBIT V-9 Example of Edge Dropoff (ROR guide)

Edge dropoffs of more than 10 mm (4 in.) have been shown to contribute to loss of control. Drivers who inadvertently drift onto the shoulder find their right wheel caught against the dropoff. This may induce overcorrecting by the driver, with resultant sudden loss of control or steering into the opposing lane. This behavior may be exacerbated when the driver is tracking a horizontal curve.

The best practice is to always retain the travel lane and shoulder at the same elevation, where they meet. Where this cannot be achieved, such as on roadways with unpaved shoulders, an alternative is to smooth the transition between the traveled way and shoulder surfaces using a wedge of pavement that allows vehicles to safely return to the roadway. For example, during pavement work in the state of Idaho, “moulding shoes” are sometimes equipped on the outside of the pavers to provide safe asphalt slopes. Georgia is also working on a 30-degree asphalt fillet. This strategy is related to strategies under the section in the ROR guide entitled “Apply Shoulder Treatments” (Strategy 15.1 A8).

Reference is usually made to edge dropoffs in the context of the boundary between the traveled-way pavement and the shoulder surface. Edge dropoffs can also occur at the boundary between the shoulder surface and roadside. Efforts should be made to prevent both types of edge dropoffs.

Particular care should be taken to minimize the potential risks of edge dropoffs in work zones. Edge dropoffs can commonly occur in work zones as the result of overlays, pavement replacement, or shoulder construction. The depth of these elevation differentials can vary from approximately 2.54 mm (1 in.), when a flexible overlay is applied, to several meters, when major reconstruction is undertaken. McDonald et al. (2002) reviewed temporary traffic control strategies in numerous states addressing edge dropoff differentials and analyzed crash data and litigation related to edge dropoffs. McDonald et al. also developed recommendations for mitigating edge dropoffs in work zones.

Summary of Effectiveness of Preventing Edge Dropoffs

The ROR guide indicates that little is known about the safety effectiveness of edgedrop treatments because it is difficult to specifically define the percentage of crashes that are caused by edge dropoffs. Regardless of the percentage, it has been proposed that a simple 45-degree-angle asphalt fillet at the lane edge would virtually eliminate this type of crash for shoulder dropoffs (Humphreys and Parham, 1994).

Strategy Attributes

The ROR crash guide presents attributes common to this strategy under Strategy 15.1 A8. The reader is directed to that guide for more detailed information related to this strategy.

Strategy 15.2 A7: Provide Skid-Resistant Pavement Surfaces (T)

General Description

Current design criteria for horizontal curves are formulated to provide comfort to the driver in tracking the curve while keeping vehicles from skidding on wet pavements. The criteria are based upon the standard curve formula that provides that a portion of the lateral acceleration developed by the vehicle will be resisted by superelevation and the remainder by tire-pavement friction. A vehicle will skid during braking and maneuvering when frictional demand exceeds the available friction at the tire-pavement interface.

Much research has been conducted to address curve operations, driver speed, vehicle paths, and safety. Harwood and Mason (1994) evaluated the margin of safety against skidding for a passenger car and truck on a horizontal curve. The margin of safety was defined as the difference between the available tire-pavement friction and the friction demand of the vehicle as it tracks the curve. The authors determined that existing design criteria provide an adequate margin of safety against vehicles skidding off the roadway, assuming vehicles do not exceed the design speed of the roadway and vehicles traverse the curve on a path that follows a constant radius equal to the radius of the curve.

The likelihood of skidding increases when these assumed conditions are violated. Several studies have shown that under real-world conditions both of these assumptions are violated to some degree (Bonneson, 2000; Glennon et al., 1985; Glennon and Weaver, 1972), with the result being that at many curve sites the assumed margin of safety may actually be overestimated. Where this is the case and there is evidence of loss of control because of skidding, several solutions are evident. Solutions may include modifications to the alignment and roadside to control speeds, changing the superelevation along the curve, and/or providing pavement surfaces with better skid resistance. Strategy 15.2 A7, however, focuses upon providing pavement surfaces with better skid resistance.

Summary of Effectiveness of Providing Better Skid-Resistant Pavement Surfaces

This strategy directly relates to Strategy 15.1 A7 in the ROR guide on skid-resistant pavement surfaces. Although further details may be found there, this section provides a brief summary of the safety effectiveness of providing better skid-resistant pavement surfaces, as presented in the ROR guide. New York State 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. 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 support earlier findings that improving the skid resistance at locations with high wet-road accident frequencies results in reductions of 50 percent for wet-road accidents and 20 percent for total accidents. While these results could be subject to some regression-tothe-mean bias, there is an indication that improving the skid resistance of pavement surfaces reduces wet-road and total accidents. Some states, including California, resurface short roadway segments such as horizontal curves with open-graded asphalt friction courses to improve skid resistance and safety.

Strategy Attributes

The ROR guide presents attributes common to this strategy, under Strategy 15.1 A7. The reader is directed to the ROR guide for more detailed information related to this strategy. The signalized intersection guide also discusses similar treatments under Strategy 17.2 G2.

In conjunction with this strategy, an agency should consider scheduling routine pavement friction tests and creating a pavement friction inventory program. Ideally, this type of program would include the entire roadway network within an agency’s jurisdiction, but at a minimum it should include the highest-volume roadways. Caltrans operates an Office of Pavement Rehabilitation, which includes a program of pavement friction inventory (http://www.dot.ca.gov/hq/esc/Translab/opr.htm). Routine pavement friction tests should be conducted on both tangent and curve sections of a highway. Research conducted by NYDOT in the late 1990s revealed that, under high-volume conditions, significant reductions in friction occurred at curves, compared with tangent sections of the same road segment treated with the same surface treatment.

Finally, drainage is an important issue to consider when implementing this strategy. As the water film thickness on the pavement increases, the likelihood of hydroplaning increases. Therefore, any drainage problems should be corrected in conjunction with this strategy. While checking for and/or correcting any drainage problems, deficiencies in the superelevation and pavement edge profiles should also be checked and improved if found deficient.

Strategy 15.2 A8: Provide Grooved Pavement (T)

General Description

Pavement grooving is a technique by which longitudinal or transverse cuts are introduced on a surface to increase skid resistance and to reduce the number of wet-weather crashes. The grooves increase skid resistance by improving the drainage characteristics of the pavement and by providing a rougher pavement surface. Several studies show that grooved pavements reduce wet-weather crashes. However, some potential adverse effects should be considered before this strategy is implemented, including the potential of increased noise pollution, accelerated wearing of pavements, and negative effects on steering.

This strategy is related to Strategy 15.2 A7 in this guide and Strategy 15.1 A7 in the ROR guide. Those strategies focus on improving skid resistance by means of changing the pavement aggregates, placing overlays, or adding texture to the pavement surface. Strategy 15.2 A8 focuses strictly on providing grooved pavement. While pavement grooving is a way to add texture to the pavement surface, its primary objective is to improve the drainage and to mitigate hydroplaning. The grooves decrease the water film thickness on a pavement surface and allow for greater tire-pavement surface interaction during adverse weather conditions. Because pavement grooving is such a unique approach to increasing the skid resistance of a pavement, it is treated separately. The section immediately following this one presents results of studies that evaluated the safety effectiveness of pavement grooving. That is followed by a section that presents attributes unique to pavement grooving that should be considered before this type of treatment is implemented.

Safety Effectiveness of Pavement Grooving

Numerous studies on the safety effectiveness of pavement grooving have been conducted, but none of these studied have controlled for regression to the mean so the results should be considered with caution. Wong (1990) performed a before-after evaluation of the effectiveness of pavement grooving based upon data from one site in California. The site was a two-lane highway with steep vertical grades and sharp horizontal curves. Based upon accident data from a 3-year before period and a 1-year after period, Wong found a 72-percent reduction in wet-pavement accidents, while only finding a reduction of about 7 percent in dry-pavement accidents. Wong concluded that pavement grooving was effective in reducing wet-pavement accidents.

Zipkes (1976) analyzed the frequency of accidents and the percentage of accidents on wet and dry pavement surfaces during a 7-year period to evaluate the effect of pavement grooving. Accident data were obtained for a 44-km (27-mi) section of highway near Geneva, Switzerland. Transverse grooves were cut into the pavement with varying groove distances over a 2-km (1.2-mi) section of highway. Grooving of the polished road surfaces reduced the hazard of accidents when drainage conditions were unfavorable. Zipkes indicated that the advantage of grooving is the reduction of water-film thickness, which leads to better contact between the tire and the road surface for the transmission of forces.

Smith and Elliott (1975) evaluated the safety effectiveness of grooving 518 lane-km (322 lane-mi) of freeways in Los Angeles, while 1,200 lane-km (750 lane-mi) of ungrooved pavement were used as a control. The analysis was conducted using 2 years of before data and 2 years of after data. Only fatal and injury accidents were included in the evaluation. Smith and Elliott found that pavement grooving resulted in a 69-percent reduction of wet-pavement accident rates. Sideswipe and hit object accidents were reduced to the largest extent. Pavement grooving did not change the dry-pavement accident rates.

Mosher (1968) synthesized results from studies conducted by state highway departments on the effects of pavement grooving. Information for the report was obtained from 17 states, including Colorado, Florida, Georgia, Idaho, Illinois, Indiana, Louisiana, Minnesota, Missouri, Nebraska, New York, Ohio, Pennsylvania, Texas, Utah, Wisconsin, and Wyoming. Some sections of highway had longitudinal grooves, while other sections had transverse grooving. Pavement grooving proved very effective, reducing crashes by 30 to 62 percent.

Farnsworth (1968) evaluated the effects of pavement grooving on five sections of California highways. Farnsworth measured the coefficients of friction before grooving and after grooving and found that pavement grooving increased the coefficients of friction, changing the friction values from below critical to above critical. Analysis of accident data revealed a reduction in wet-pavement accidents at each of the sites.

NYDOT evaluated the safety effectiveness of pavement grooving based on the installation of grooves at 41 sites. NYDOT found that wet-road accidents were reduced by 55 percent, and total accidents (dry and wet) were reduced by 23 percent. The results were statistically significant at the 95th percentile. Regression to the mean was not addressed in the analysis.

Strategy Attributes

Pavement grooving involves making several shallow cuts of a uniform depth, width, and shape in the surface of the pavement (Mosher, 1968). Grooves may be cut longitudinally along the pavement (parallel to the direction of travel) or in the transverse direction (perpendicular to the direction of travel). Transverse grooving has been used to a lesser extent than longitudinal grooving, partially because most grooving equipment lends itself more readily to placing grooves parallel to the roadway. Grooves cut in the longitudinal direction have proven most effective in increasing directional control of the vehicle, while transverse grooving is most effective where vehicles make frequent stops, such as intersections, crosswalks, and toll booths. When pavements are grooved, it is important that the pavement contain nonpolishing aggregate.

While studies have indicated that pavement grooving reduces wet-pavement accidents, there have been several concerns associated with pavement grooving (Mosher, 1968). One concern has been the effect that pavement grooving has on the durability of various pavement types. For example, one of the most frequently asked questions by engineers in northern climates is, “What will water freezing in the grooves do to the concrete pavement?” In an examination of grooved pavement in Minnesota after one winter, there appeared to be no deterioration in the grooved pavement because of the freeze-thaw cycles. Concern also has been expressed about grooves in asphalt pavement losing their effectiveness because the material can be flexible enough to “flow” back together, particularly during hot weather. This phenomenon has been observed under certain conditions with a fairly new asphalt pavement or with a pavement with low aggregate content. Concern has also been expressed over the loss of effectiveness because of grooved pavements wearing down under hightraffic conditions.

Complaints also have been received that longitudinal grooves adversely affect the steering of certain automobiles and motorcycles. In general, no severe problems have been encountered. This finding is supported by research conducted by Martinez (1977), who studied the effects of pavement grooving on friction, braking, and vehicle control by computer simulation and full-scale testing. Martinez considered automobiles, motorcycles, and automobile and towed-vehicle combinations in his evaluation.

In Iowa, residents living adjacent to I-380 near Cedar Rapids complained that transverse grooving was the cause of an especially annoying tonal characteristic within the traffic noise (Ridnour and Schaaf, 1987). As a result of the complaints, the surface texture of a section of I-380 was modified. The transverse grooving was replaced with longitudinal grooving. A before-after analysis of the traffic noise levels showed that the surface modification lowered overall traffic noise levels by reducing a high-frequency component of the traffic noise spectrum.

Strategy 15.2 A9: Provide Lighting of the Curve (T)

General Description

Approximately 51 percent (4,977) of the 9,791 fatal crashes that occurred at horizontal curves in 2002 took place during nighttime hours. To a large extent, these crashes may be attributed to reduced visibility at night.

There is evidence to show that providing fixed-source lighting in urban and suburban areas, where there are concentrations of pedestrians and intersectional interferences, reduces nighttime crashes. The need for lighting on streets and highways in rural areas is much less than on streets and highways on urban areas. The need for lighting on rural highways is seldom justified except in critical areas, such as sharp curves (AASHTO, 2001).

Exhibit V-10
Strategy Attributes for Providing Lighting of the Curve (T)

Technical Attributes
Target	Drivers of vehicles approaching a horizontal curve who have difficulty seeing the curve during non-daylight hours.
Expected Effectiveness	The expected safety effectiveness of providing lighting at a horizontal curve is difficult to assess. Providing lighting at a horizontal curve helps to enhance the driver’s available sight distance during nighttime conditions. In addition, lighting at a horizontal curve helps to provide advance warning of unexpected changes in horizontal alignment, and it helps to enhance delineation along a curve, particularly during adverse weather conditions. However, factors other than ambient light may be involved.
Keys to Success	Many crashes occur during nighttime hours at horizontal curves. However, not all of these crashes are attributed to reduced visibility. It is important to diagnose the problem and determine if the accident pattern is correctable by providing lighting.
Potential Difficulties	There are two potential difficulties associated with providing lighting at horizontal curves. First, the cost might be prohibitive, especially in rural areas. Second, luminaire supports (i.e., poles) are additional fixed objects that a vehicle could strike when it leaves the roadway. When a decision is made to provide lighting at a curve, the luminaire supports should be located in the least hazardous locations along the curve. Consideration should be given to the use of break-away poles. See the guide for utility poles for more information on minimizing the risk of poles.
Appropriate Measures and Data	Process measures of program effectiveness would include the number of horizontal curves where lighting was provided. Impact measures include the number and rate of nighttime crashes and the ratio of day-to-night crashes.
Associated Needs	A highway lighting plan would have to be developed when implementing this strategy. It is anticipated that the state DOT or local roadway agency would develop its own highway lighting plans from its own specifications or AASHTO’s Informational Guide for Roadway Lighting.
Organizational and Institutional Attributes
Organizational, Institutional and Policy Issues	Providing power to certain locations may require installation of power lines across multiple government jurisdictions. This will require the cooperation and coordination of multiple government agencies and the power company.
Issues Affecting Implementation Time	The availability of a power source at the horizontal curve will affect the implementation time. In urban and suburban areas, this is not a significant matter, but in rural areas, if overhead or underground power lines do not run along the right-of-way (ROW) near the location of the curve, providing a cost-effective approach for supplying electricity at the horizontal curve will increase the implementation time.
Costs Involved	There are several types of costs associated with providing lighting of a curve, including the cost of providing a permanent source of power to the location, the cost for the luminaire supports (i.e., poles), and the cost for routinely replacing the bulbs and maintenance of the luminaire supports. The cost for providing a permanent source of power to the location could be high if the curve is located in a remote rural area. In some cases, solar-powered lighting may be appropriate. An example is shown in Appendix 2. In many cases, local municipalities may be required to take responsibility for operational and maintenance costs associated with lighting. Some municipalities may be reluctant to take over these responsibilities, even if separate funds (i.e., state funds) are provided for capital costs.
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.
Legislative Needs	None identified.
Other Key Attributes
None identified.

Strategy 15.2 A10: Provide Dynamic Curve Warning System (T)

General Description

The purpose of this strategy is to reduce the speed of high-speed vehicles on their approach and as they navigate through a horizontal curve. A typical system combines a radar device with a variable message sign. The system measures the speeds of approaching vehicles and provides messages to drivers who are traveling at excessive speeds to slow down to a recommended, or advisory, speed (Exhibit V-11). Dynamic curve warning systems can also incorporate cameras to provide visual surveillance of curves. These systems can be developed using off-the-shelf technology. The main hypotheses regarding this type of strategy are that a dynamic warning device has a much greater effect on high-speed vehicles than a static curve warning sign and that the dynamic system significantly improves the ability of highspeed vehicles to successfully navigate through the curve.

EXHIBIT V-11 Sequence of Messages on Dynamic Curve Warning System in California (Photo Provided by Caltrans)

Several dynamic curve warning systems have also been deployed specifically to reduce the likelihood of a truck rollover crash. In 1998, 207 trucks were involved in fatal rollover accidents on the U.S. highway system (Baker et al., 2001). Truck rollover accidents often occur at exit ramps and at tight curves that require a more reduced speed than the normal travel speed on the freeway. Therefore, many of the dynamic curve warning systems designed to reduce rollover crashes have been deployed at freeway exit ramps. Exhibit V-12 illustrates such a system.

EXHIBIT V-12 Freeway Ramp Example of Dynamic Curve Warning System (McGee and Strickland, 1994)

Exhibit V-13
Strategy Attributes for Dynamic Curve Warning System (T)

Technical Attributes
Target	Drivers of vehicles approaching a curve at an undesirable speed.
Expected Effectiveness	The safety effectiveness of dynamic curve warning systems is not completely known because very few systems have been installed, but preliminary evaluations of several systems are promising. Evaluation results from three studies are presented below. Three truck rollover warning systems were installed at three ramps on the Capital Beltway (I-495) in Virginia and Maryland (Strickland and McGee, 1996). The objective of these systems is to identify a truck of a certain type that is traveling toward a curved ramp whose speed is likely to approach or exceed the rollover threshold speed. The device is then to warn the driver of the truck to slow down before reaching the curve. The primary components of the system include weigh in-motion detectors, speed loop detectors, height detectors, fiber-optic signs, and controllers to operate the systems. Based on an analysis of speed data, it was concluded that all three systems significantly impacted truck speeds and that the systems caused truck drivers to reduce their speeds before entering ramps, when their speed was exceeding the maximum safe speed. Before installing the rollover warning systems, a combined total of 10 truck rollover accidents occurred at the sites. After 3 years of operation, no rollover crashes were reported. In Minnesota, a dynamic curve warning system was installed on the approach to a 4-degree curve along a county highway (Preston and Schoenecker, 1999). The section of highway is a two-lane road with a posted speed limit of 89 km/h (55 mph), which is frequently used by unfamiliar drivers. A field test was conducted over a 4-day period to evaluate the effectiveness of the system. Vehicle speed and navigation measures were used to evaluate the system. The general effect of the system on vehicle speeds was relatively small. However, the dynamic system had a much greater effect on high-speed vehicles than the static curve warning sign. In addition, the dynamic system significantly improved the ability of high-speed vehicles to successfully navigate the curve. A 2000 report researches the effects of dynamic curve warning signs installed on five curves along a rural stretch of the California Interstate System that experienced relatively low volumes of traffic (Tribbett et al., 2000). The objective was to determine whether signs displaying information concerning the upcoming curve and the driver’s actual speed would have any effect on the driver’s approach to handling the horizontal curve. Analysis showed that significant truck speed reduction was found for three of the five curves and significant passenger vehicle speed reduction was observed at two of the five curves after the warning systems had been installed.
Key to Success	The key to a successful dynamic curve warning system is identifying vehicles that are actually traveling above the maximum safe speed and conveying this message to the drivers soon enough that they can adjust their speeds before reaching the curve. When a dynamic warning system provides a false warning to motorists to reduce their speeds, the system loses credibility. A false warning would include a message to motorists traveling at a safe speed that they should reduce their speed to successfully navigate the curve.
Potential Difficulties	Determining the maximum safe speed for a particular vehicle entering a horizontal curve is a very difficult task in the field. Many factors come into play, such as loads, suspension, vehicle size and configuration, and quality of the tires. Many algorithms, ranging from the very simple to the very complex, can be used to determine if vehicles are exceeding the maximum safe speed. Another issue to consider is adverse weather conditions. The target speed, or maximum recommended safe speed, may be different given the weather conditions. The challenge is developing a system sophisticated enough to minimize false readings while still being costeffective to deploy. As systems become more complex, the cost of components increases. Another potential difficulty that could be associated with deploying a dynamic curve warning system is identifying the proper location for this type of treatment. This issue involves several aspects. First, an agency must determine that the types of accidents occurring at a particular curve are correctable with this treatment. Second, the geometrics near the curve, including both the horizontal and vertical alignments, must be compatible with such a system. Sight lines must be available for radar equipment and possible video equipment. Weigh-in-motion devices may need to be installed as well. Third, if the curve warning system is for an off ramp, difficulties arise in identifying those vehicles that are exiting. Thus, careful consideration should be given to locating dynamic curve warning systems. It also must be remembered that the placement of these devices at the roadside can result in a fixed-object hazard for vehicles that may run off the road. A power source must also be available.
Appropriate Measures and Data	In the evaluation of these systems, process measures would include the number of curves treated or the number of systems deployed. Impact measures involve comparison of crash frequencies or rates (with the study appropriately designed) for the before period and after period. A particular comparison of interest might be the number of rollover-related crashes in the before period to the number in the after period. The change in speed for vehicles entering selected curves would also be an impact measure. Consideration should be given to targeting the highest quartile speed group of vehicles when evaluating the effectiveness of the system based on speed. A surrogate measure might also include the number of encroachments onto the shoulder along the curve.
Associated Needs	Depending on the complexity of the system, many highway agencies may not have the technical expertise to develop and deploy a dynamic curve warning system. Therefore, many agencies will have to contract with an outside consultant to implement this strategy.
Organizational and Institutional Attributes
Organizational, Institutional and Policy Issues	Proprietary issues concerning ownership of the software and actual algorithms could arise during contractual arrangements.
Issues Affecting Implementation Time	The complexity of the system will affect the implementation time. For example, the primary components of the system installed in Minnesota were a radar device and a dynamic message sign. The systems installed in Virginia and Maryland involved weigh-in-motion devices, loop detectors, and height detectors. The algorithm for identifying vehicles exceeding the maximum safe speed was much more complicated for the system installed in Virginia than for the system used in Minnesota. Simple systems can be installed in a short timeframe, while the more complex systems might take a year or two from conception to implementation.
Costs Involved	The complexity of the system will affect the cost. Further details are provided in Appendix 3.
Training and Other Personnel Needs	There appear to be no special personnel or training needs for implementing this strategy because most of the work will be conducted by an outside contractor if the system is complex. Agencies should have the technical expertise to develop and install simple systems.
Legislative Needs	None identified.
Other Key Attributes
	In addition to the systems identified above, dynamic curve warning systems have been installed in several other states, including Pennsylvania, Colorado, and Missouri. Similar systems have also been installed in the United Kingdom.

Strategy 15.2 A11: Widen the Roadway (P)

General Description

It is common practice to widen the traveled way on horizontal curves to make operating conditions on curves comparable to those on tangents. As noted in the AASHTO (2001) policy, widening the traveled way on horizontal curves is necessary for one of two reasons, either (1) the design vehicle occupies a greater width in negotiating the curve because of offtracking or (2) drivers experience difficulty in steering their vehicles along the center of the lane. Roadway widening, however, can entail more than just widening the travel lanes. It can include widening the shoulders, providing shoulders where none previously existed, providing a buffer zone in the middle of the roadway, or various combinations of the above. By widening the traveled way, drivers have more space within the lane to maneuver their vehicles through the curve, allowing more room for driver error without serious consequences By widening the shoulders or providing a shoulder where one previously did not exist, drivers have more recovery area to regain control of their errant vehicles before encroaching on the roadside.

The section immediately following this provides a brief summary of the safety effectiveness of widening the roadway as presented in the ROR guide.

Summary of Effectiveness of Roadway Widening

Two strategies in the ROR guide pertain to roadway widening: Strategies 15.1 A5 (improved highway geometry for horizontal curves) and 15.1 A8 (apply shoulder treatments). These may be referenced for further details.

Strategy 15.1 A5, on improving highway geometry for horizontal curves, provides accident reduction factors for widening lanes and/or shoulders on horizontal curves. Widening a lane may reduce accidents by 5 to 21 percent. Widening a paved shoulder may reduce accidents by 4 to 33 percent, and widening unpaved shoulders may reduce accidents by 3 to 29 percent.

Strategy 15.1 A8, on applying shoulder treatments, provides one set of accident modification factors for widening a paved shoulder on a two-lane rural highway and a second set of accident modification factors for various shoulder types and widths. The accident modification factors were developed by a panel of experts charged with developing prediction models on the expected safety performance of rural two-lane highways. The accident modification factors for widening a paved shoulder width vary as a function of shoulder width and average daily traffic. The base case used is a 1.8-m (6-ft) paved shoulder, and the accident modification factors range from 0.87 to 1.50. Regarding the accident modification factors for shoulder type, the base case is a paved shoulder. Depending on the shoulder width, the accident modification factors range from 1.00 to 1.03 for gravel shoulders, from 1.00 to 1.07 for composite shoulders, and from 1.00 to 1.14 for turf shoulders.

Strategy Attributes

The ROR guide presents attributes common to this strategy under Strategies 15.1 A5 and 15.1 A8. The reader is directed to that guide for more detailed information related to this strategy.

Finally, there is concern that widening the roadway may increase operating speeds. Because speed is such a critical factor related to safety at horizontal curves, roadway widening may worsen safety.

Strategy 15.2 A12: Improve or Restore Superelevation (P)

General Description

Superelevation is one of the key geometric elements of curve design. Designers select a superelevation rate consistent with the design speed, the selected curve radius, and their jurisdiction’s policy for maximum superelevation. Superelevation works with friction between the tires and pavement to counteract the forces on the vehicle associated with cornering.

Many curves may have inadequate superelevation because of vehicles traveling at higher speeds than were originally designed for, because of loss of effective superelevation after resurfacing, or because of changes in design policy after the curve was originally constructed. For whatever reason, curves with inadequate superelevation may pose safety problems, particularly if the actual superelevation is less than the optimal superelevation as recommended by AASHTO policy (AASHTO, 2001).

Accident prediction models indicate that inadequate superelevation increases curve accidents (Zegeer et al., 1992). There is no evidence, however, that safety is adversely affected along a curve where the actual superelevation is greater than that recommended by AASHTO policy. Therefore, research results indicate that safety can be enhanced when the superelevation is improved or restored along curves where the actual superelevation is less than the optimal superelevation. The following section presents the safety effectiveness of improving or restoring superelevation along curves. The discussion is then concluded with a section that presents other issues relevant to this strategy.

Safety Effectiveness of Improving or Restoring Superelevation

Improving the superelevation of a curve can reduce curve accidents where there is a superelevation deficiency (Zegeer et al., 1991). Superelevation deficiency is the numerical difference between the optimal superelevation (as determined from AASHTO policy) and the actual superelevation of a given curve. Based on estimates from Zegeer et al. (1991), an improvement of 0.01 to 0.019 in superelevation (e.g., increasing superelevation from 0.04 to 0.05 to meet AASHTO policy) would be expected to yield an accident reduction of 5 percent. An improvement of 0.02 or greater in superelevation would be expected to yield an accident reduction of 10 percent.

In 2000, an expert panel used the Zegeer work to develop accident modification factors (AMFs) for the superelevation of a horizontal curve on two-lane highways (Harwood et al., 2000). The following relationships were developed based on the expert panel’s judgement:

	AMF = 1.00 for SD < 0.01	(1)
	AMF = 1.00 + 6(SD - 0.01) for 0.01 <= SD < 0.02	(2)
	AMF = 1.06 + 3(SD - 0.02) for SD >= 0.02	(3)
	where   SD = superelevation deficiency.

These relationships indicate that there is no effect on safety until the superelevation deficiency reaches 0.01, which is consistent with the Zegeer work.

Strategy Attributes

EXHIBIT V-14 Example AMFs for Superelevation Deficiency
SD	AMF
0.009	1.00
0.0199	1.06
0.0299	1.09
0.0399	1.12

During routine pavement projects, deficiencies in superelevation should be addressed (Zegeer et al.). There are several other issues related to superelevation that should be considered during routine pavement projects and during original construction. First, it is important to limit the slope break between the elevated edge of pavement and the adjacent shoulder. This can be achieved by designing the shoulder to be sloped upward at approximately the same rate as, or at a lesser rate than, the superelevated traveled way or by flattening the shoulder on the outside of the curve. AASHTO (2001) provides guidance on maximum recommended algebraic differences between the traveled way and the shoulder slopes.

The second issue is proper transition from the normal cross slope along the tangent to the fully superelevated cross slope along the curve. For reasons of safety and comfort, the rotation of the pavement should be effected over a length that is sufficient to make such rotation imperceptible to drivers. Normal practice, in the absence of spiral transition curves, is to begin rotating the pavement along the tangent section before the curve and not attain full rotation until into the curve. A portion of the superelevation runoff is typically located on the tangent, in advance of the point of curvature, to minimize peak lateral accelerations and side friction demand. The proportion of the runoff length placed on the tangent varies from 60 to 80 percent, with many agencies using the 2/3–1/3 rule (placing 2/3, or 67 percent, of the runoff length on the tangent). Bonneson (2000) re-evaluated the approach to horizontal curve superelevation/transition design and determined that placing 70 percent of the superelevation runoff on the tangent and 30 percent on the curve was optimal.

Finally, care should be given to provide proper drainage when improving or restoring the superelevation along a curve. The combination of the control line longitudinal profile and superelevation can produce unintended flat spots along the roadway if care is not taken during design of the transition.

Strategy 15.2 A13: Modify Horizontal Alignment (P)

General Description

This strategy is the longest-term, highest-cost alternative considered for improving the safety of a horizontal curve because it usually involves total reconstruction of the roadway. It may also require acquisition of additional right-of-way and an environmental review.

There are several ways in which the horizontal alignment of a roadway may be modified to improve safety, including

• Increasing the radius of a horizontal curve,
• Providing spiral transition curves, and
• Eliminating compound curves.

These modification approaches are addressed below.

Safety Effectiveness of Increasing the Radius of a Horizontal Curve

Increasing the radius of a horizontal curve can be very effective in improving the safety performance of the curve. This strategy is also covered under Strategy 15.1 A5 (improved highway geometry for horizontal curves) in the ROR guide. The ROR crash guide presents a table on the percent reduction of total crashes on a two-lane rural highway that would be expected after flattening a curve. The table is based on research conducted by Zegeer et al. (1992) and shows that increasing the radius of curvature can reduce total curve-related crashes by up to 80 percent. An expert panel used the work of Zegeer et al. to develop an accident modification factor for horizontal curvature on rural two-lane highways (see Equation 4). The accident modification factor is a function of the length of the curve, the radius of the curve, and the presence or absence of a spiral.

		(4)
	where   Lc = length of horizontal curve (mi),   R = radius of curvature (ft), and   S = 1 if spiral transition curve is present and   0 if spiral transition curve is not present.

Safety Effectiveness of Providing Spiral Transition Curves

A spiral transition curve is a horizontal curve with a continuously changing radius that connects a tangent and a circular curve or two circular curves of different radii. A spiral provides a smooth transition between a tangent section and an adjacent circular curve. The smoother transition from tangent to curve and curve to tangent results in the lateral force increasing and decreasing gradually as a vehicle enters and departs the curve. This is intended to minimize encroachment on adjacent traffic lanes and to promote uniformity in speed (AASHTO, 2001).

Research has shown that drivers on unspiraled curves track a path radius substantially sharper than the designed radius. This is primarily because it is not possible for drivers to instantaneously change their path radius from tangent to curve, and, once on the curve, they must “overcorrect” to stay on the roadway. Providing spirals affords the driver the means of tracking a curve that fits the designed alignment.

Council (1998) and Zegeer et al. (1992) reported that spiral transition curves are effective in reducing crashes. The findings are based on studies of spiraled versus unspiraled curves in one state.

An expert panel that developed the accident modification factor for horizontal curvature on rural two-lane highways (Equation 4) judged that there is sufficient evidence to conclude that the presence of spiral transitions on horizontal curves improves safety. The negative sign associated with the spiral variable effectively reduces the value of the accident modification factor, indicating that the presence of a spiral transition at a horizontal curve improves the safety of the curve. An example is presented below that reveals in more detail the effects of curve length, curve radius, and spiral transitions on safety performance.

Exhibit V-15
Example Values for AMFs for Horizontal Curves with and without Spiral Transitions

Degree of Curve	Radius of Curve ft	Central Angle Degrees	Length of Curve mi	AMFs
				w/spiral	w/o spiral
38	150	150	0.07	5.5	5.6
11	500	20	0.03	3.9	4.1
6	1000	20	0.07	1.7	1.8
3	2000	20	0.13	1.1	1.2
2	3000	20	0.20	1.0	1.1

Safety Effectiveness of Eliminating Compound Curves

Compound circular curves are sometimes advantageous in providing desirable shapes of curves. However, although no quantitative comparisons have been made between the safety at simple circular curves and the safety at compound curves, agencies should be cautious of using compound curves, particularly if the radius of the first curve is significantly greater than the radius of the following curve. An abrupt change in alignment requires considerable steering effort by motorists to travel safely through the successive curves.

If an agency permits the use of compound curves, the designs should meet AASHTO (2001) guidelines. AASHTO recommends that for compound curves on open highways, the ratio of the flatter radius to the sharper radius should not exceed 1.5:1, and on ramps the ratio may be greater, possibly as great at 2:1. Wherever practical, however, smaller differences in radii should be used.

Strategy Attributes

In general, whether the curve is isolated between two long tangents or is located along a stretch of curvilinear roadway, the horizontal alignment should be designed to meet a driver’s expectation. When an alignment fails to meet a driver’s expectation, the alignment should be modified accordingly. Speed profile models may be used to evaluate the conformance of a highway’s geometry with driver expectancy. Fitzpatrick et al. (2000b) have developed a speed profile model that may be used to evaluate the design consistency of two-lane rural roads. This model was developed for use in the Interactive Highway Safety Design Model (IHSDM) (http://www.fhwa.dot.gov/ihsdm/index.htm).

Strategy 15.2 A14: Install Automated Anti-Icing Systems (T)

General Description

Automated anti-icing systems are a potentially effective tool for keeping roadway surfaces clear of ice and safe for travel in areas of the country with severe winter climates. Anti-icing involves pretreatment of the roadway surface with chemicals before a winter storm arrives, as opposed to deicing, which involves treatment of the roadway surface during or after the storm, when ice may already have formed. The most common automated anti-icing system now in use is one that uses a series of spray-nozzles connected to a chemical storage tank. Using a pump system, a liquid anti-icing agent is distributed along a roadway segment using the nozzles that can be embedded into the pavement or placed along the edges of the road. The system can either be fully automated, relying wholly on sensors in the area to determine the need of an application of anti-icing chemicals, or semi-automated, where the system can be engaged by someone from a remote location in response to a sensor indication. A potential advantage is that automated anti-icing system can be engaged immediately when appropriate conditions are detected. Existing approaches for anti-icing involve a delay until staff can be called out and equipment deployed. Furthermore, truck-based systems treat locations in sequence according to a routing or priority scheme; automated systems can be activated at all appropriate locations simultaneously.

While not yet in widespread use, automated anti-icing systems are beginning to be used in place of exiting anti-icing systems that involved motorized vehicles traveling along roadway lanes while distributing anti-icing agents. Seventeen states have installed anti-icing systems at selected locations. The most prevalent locations for current automated anti-icing systems are on bridges and overpasses where deck surfaces are prone to ice formations sooner than adjoining road sections. One system in California is currently being planned at a location on a horizontal curve. (See Appendix 4 for more detailed information on this system in California.) Horizontal curves are logical locations for anti-icing treatment because tirepavement friction is critical to vehicle control at horizontal curves.

Many crashes each year result from the vehicle loss of control while traveling on icy roads. Anti-icing systems that have been installed have shown benefit/cost ratios in the range from 1.8 to 3.4 and reductions in the frequency of wintertime accidents from 25 to 100 percent (Friar and Decker, 1999; Barrett and Pigman, 2001; Khattak and Pesti, 2003). However, despite the positive safety evaluations, this strategy is listed as “tried” primarily because this technology has not been in use for a long period of time, nor have the studies specifically addressed installations along horizontal curves.

Exhibit V-16
Strategy Attributes for Installing Automated Anti-Icing Systems at Horizontal Curves (T)

Technical Attributes
Target	Accidents involving skidding on horizontal curves because of icy road surface conditions.
Expected Effectiveness	Recent evaluations have reported benefit/cost ratios in the range of 1.8 to 3.4 and reductions in accident frequency in the range of 25 to 100 percent. However, experience with these systems is limited, and the systems evaluated have not been deployed on horizontal curves. Further research to quantify the safety benefits of these systems on roadways in general, and specifically on horizontal curves, is desirable.
Key to Success	A key to success in the use of automated anti-icing systems is their use at locations with the greatest need. Appropriate locations for their application include sites with high traffic volumes, high wintertime accident rates, and/or isolated locations that are difficult to reach in bad weather (Khattak and Pesti, 2003).
Potential Difficulties	In a report by Barrett and Pigman (2001), certain difficulties were listed from an automated anti-icing system installed on a Kentucky Interstate bridge in October 1997. For the first couple of seasons of use, a certain amount of system debugging had to take place. However, when the logistics had been corrected, the system was found to be quite self-sufficient. There were other states that observed their systems operating more efficiently in non-snow events or light snow storms. Some systems were also found to work better in semi-automatic mode. In this case, the system can be turned on from a remote location as personnel see fit as opposed to relying wholly upon the field sensors to detect a need for anti-icing measures. A potential difficulty can also be found in retrofitting existing roads with automated anti-icing systems. The most common system being used is one that distributes a liquid anti-icing agent onto the roadway. This is set up with spouts distributed along the length of a roadway segment, either slightly elevated on the roadside or embedded into the road’s surface. While not impracticable, the difficulties and expense of this installation should be taken into account. Another potential difficulty was noted by Stowe (2001). Automated systems must be constantly monitored because the system could malfunction due to a number of circumstances, such as external debris or solids in the chemical spray that plug the nozzles of the system. If system malfunction were to occur, ice could begin to form on the roadway. System malfunction must be detectable so that appropriate maintenance can restore system operation.
Appropriate Measures and Data	The process measures of program effectiveness would include the length of roadway and number of horizontal curves where automated anti-icing systems were successfully installed. The impact measures would require a study of the site’s wintertime crash statistics before and after the installation of the automated anti-icing system. A concomitant measure of effectiveness would be the cost savings to highway agencies by not having to deploy personnel, equipment, and materials.
Associated Needs	Because automated anti-icing systems have not been extensively used on horizontal curves, more information is needed as to whether it is actually feasible to use current systems in that environment. It may also be necessary to evaluate the effect that the dispensed chemicals will have on the surrounding environment.
Organizational and Institutional Attributes
Organizational, Institutional and Policy Issues	The implementation of this strategy will be done at the discretion of the state and local transportation agencies. It does not appear that there will be any issues arising between organizations or institutions. As this is a maintenance function that is administrative in nature, care should be taken regarding the selection and documentation of sites to be treated. The agency will want to prepare for potential tort claims for curves in which such treatment is not provided. It will be important to upgrade maintenance protocols to ensure that the performance of the systems is maintained during inclement weather, both for the safety of the traveling public and to avoid exposure to liability. There is a definite need for a full evaluation of current winter maintenance strategies before an agency decides on an appropriate role for automated anti-icing systems.
Issues Affecting Implementation Time	The major issues affecting the time required to implement automated anti-icing systems are the time required to prioritize roadway segments for installation of the systems and the time required for installation. Selection of locations for automated anti-icing systems should be based on consideration of wintertime accident rates, traffic volumes, horizontal alignments, and distances from maintenance yards to determine which areas are in the greatest need for such systems. After priorities have been set, then the actual system design and installation can take place. The time needed for installation is influenced by the length of road to be treated by the automated system and the type of system chosen.
Costs Involved	For an automated anti-icing system installed in Kentucky (Barrett & Pigman, 2001), the approximate cost was $65,000. This included roadway coverage 183 m (600 ft) long and 3 lanes wide. This cost estimate does not include the cost of the road weather information system (RWIS), which is needed to determine when anti-icing becomes necessary. A detailed discussion of RWIS, including a cost-benefit comparison, may be found at http://www.its.dot.gov/JPODOCS/REPTS_TE/13660.html#_Toc535648483. For information about a Vermont DOT installation of an RWIS, see http://www.aot.state.vt.us/matres/Documents/ACROBAT.pdf/web953.pdf. Another source (Stowe, 2001) has estimated installation cost for a 1,320-m (4,330-ft), two-lane road segment at $599,500. However, this estimate is more inclusive in that it covers the automated anti-icing system, the RWIS, and the design engineering fees involved. In addition to this amount, it is estimated that the annual operations and maintenance costs will be roughly $32,800. This figure includes materials, power, communications, maintenance, weather forecasting, and training.
Training and Other Personnel Needs	There does not appear to be any need for specialized personnel to implement this strategy, but staff need to be trained on the operation and maintenance of the system.
Legislative Needs	None identified.
Other Key Attributes
	There has been only limited use of anti-icing systems by highway agencies. Therefore, it is not possible to identify problems or benefits associated with a larger network of such systems operating in a given area. A better understanding of the application of anti-icing systems at horizontal curves is also needed.

Strategy 15.2 A15: Prohibit/Restrict Trucks with Very Long Semitrailers on Roads with Horizontal Curves that Cannot Accommodate Truck Offtracking (T)

General Description

Longer trucks, particularly single-trailer combination trucks with longer semitrailers, may have difficulty negotiating sharp horizontal curves because they may encroach on an adjacent lane or shoulder because of vehicle offtracking. Vehicle offtracking is the phenomenon in which the rear axles of a truck do not follow the same path as the front axle; at lower speeds, the rear axles of the truck typically track to the inside of the front axle path.

The magnitude of vehicle offtracking is the amount of radial distance displaced from the center of the first axle to the center of the rear axle as a vehicle is making a turn (see Exhibit V-17). Offtracking is a primary determinant of the amount of space a truck or other large vehicle occupies when executing a turning movement; this space, known as the swept path width, is the maximum width of the envelope defined by the front outside corner and the rear inside corner of the truck as it turns (Exhibit V-18). The maximum offtracking and swept path width generally occurs when a vehicle is turning at a low speed. Increasing vehicle speed gradually brings the rear of the truck back toward the path defined by the front axles of the truck. At very high speeds, the rear of the truck can actually offtrack toward the outside of the turn. However, low-speed offtracking is a consideration in the design of any roadway or intersection because traffic or environmental conditions will normally require low-speed travel sometimes on any roadway. For a more complete discussion of vehicle offtracking considerations in roadway design, see NCHRP Report 505 (Harwood et al., 2003).

The greatest concern in vehicle offtracking relevant to horizontal curves is the operation of tractor-semitrailer combination trucks with long semitrailers on roadways with sharp horizontal curvature. The distance from the point of connection of the trailer with the tractor (kingpin) to its rear axle is a critical criterion. A number of states restrict the kingpin-to-rearaxle distance for tractor-semitrailer combinations with semitrailers over 14.6 m (48 ft) in length; on many semitrailers, the rear axles of the trailer can be moved forward to comply with kingpin-to-rear-axle distance limitations. In states where longer kingpin-to-rear-axle distances

EXHIBIT V-17 Illustration of Truck Offtracking (Harwood et al., 2003)

EXHIBIT V-18 Illustration of Swept Path Width (Harwood et al., 2003)

for trucks are permitted, it may be desirable to prohibit or restrict trucks with kingpin-to-rearaxle distances that exceed a specified threshold from operating on specific facilities.

California has an active program of identifying roadways with horizontal curves that cannot accommodate trucks with longer kingpin-to-rear-axle distances and establishing appropriate truck advisory restrictions on particular roads (Caltrans, 1989). This program has been in place for nearly 20 years. California establishes these restrictions based on the distance from the kingpin to the center of the rearmost axle of the trailer, known as the kingpin-to-center-of-rearaxle (KCRA) distance. Most other states base their restrictions on the distance from the kingpin to the center of the rear tandem axle set, known as the kingpin-to-rear-tandem (KCRT) distance. The KCRT distance is generally approximately 0.6 m (2 ft) shorter than the KCRA distance.

Along 5,414 km (3,364 mi) or 22 percent of California’s 24,400-km (15,166-mi) state highway system, the California Department of Transportation has established advisory restrictions for trucks with KCRA distances that exceed 12 m (40 ft). This portion of the highway system was selected for restriction based on an analysis of vehicle offtracking on the horizontal curves’ geometrics actually present on those roadways. These restrictions were not based on established accident patterns on the roadways in question, but rather on the potential for collisions when large trucks encroach on adjacent lanes or shoulders.

In California, advisory restrictions on truck use of particular facilities are implemented by signing on the roads in question and maps for truckers published by the California Department of Transportation. An example of a sign used to inform truckers of the restriction is illustrated in Exhibit V-19.

Similar restrictions have been established for motorcoach operators in California, and system maps intended specifically for motorcoach operators are also published.

EXHIBIT V-19 California Truck Advisory Sign (http://www.dot.ca.gov/hq/traffops/trucks/trucksize/fs-trkrouts.htm)

Exhibit V-20
Strategy Attributes for Prohibiting/Restricting Trucks and/or Other Large Vehicles on Roadways with Horizontal Curve Geometrics that Cannot Accommodate Vehicle Offtracking (T)

Technical Attributes
Target	Accidents related to vehicle encroachments on adjacent lanes or shoulders. The resulting restrictions are targeted at drivers of trucks with long semitrailers, or other large vehicles, on roadways with horizontal curve geometrics that may not accommodate their vehicles within the travel lane.
Expected Effectiveness	There are no studies available on the effectiveness of this strategy, such as estimated accident reductions. The strategy has been implemented in one state not because of an existing accident pattern, but as a proactive measure to prevent such an accident pattern from developing were trucks with longer semitrailers permitted to operate on such facilities with restrictive horizontal geometrics.
Key to Success	The keys to success for this strategy are (1) an effective process to determine whether the geometrics of particular roadways can accommodate vehicles with particular offtracking characteristics; (2) an effective rulemaking process that allows truckers, motorcoach operators, and affected communities an opportunity for input into the decision-making process; (3) the availability of appropriate alternative routes for truck or motorcoach operation; and (4) the use of both signing and published maps to inform truck and motorcoach operators of the restrictions.
Potential Difficulties	See legislative needs.
Appropriate Measures and Data	Process measures of program effectiveness would include the number of miles of road, or number of curves, where trucks and/or other large vehicles were prohibited because of unsafe geometrical properties. Impact measures include the number of crashes because of trucks or other large vehicles encroaching upon other lanes or running off the road because there is not enough space to accommodate vehicle offtracking. Such impact measures are difficult to determine because restrictions are normally established proactively to keep such accident patterns from developing.
Associated Needs	Analysis of appropriate restrictions requires accurate data on the actual horizontal curve geometrics on the roadway system.
Organizational and Institutional Attributes
Organizational, Institutional and Policy Issues	State and local highway agencies would have to adopt policies that would include criteria and methods to be used to determine where restrictions of usage by large trucks should be applied. In addition, funding and personnel must be committed to enforce these policies.
Issues Affecting Implementation Time	State and local highway agencies would have to adopt policies that would include criteria and methods to be used to determine where restrictions of usage by large trucks should be applied. In addition, funding and personnel must be committed to enforce these policies.
Costs Involved	The cost of implementing this strategy is primarily the cost of staff time needed to conduct site-specific analyses, to conduct an appropriate public involvement process, to establish appropriate signing, and to publish appropriate maps.
Training and Other Personnel Needs	Law enforcement personnel need training on the appropriate enforcement of this strategy.
Legislative Needs	Many agencies lack legal authority to restrict truck use on their facilities or must comply with specific legislative requirements in establishing truck restrictions. Implementation of this strategy in some jurisdictions would require new legislation. Another potential approach is to make the restrictions advisory, rather than regulatory, as was done in California. It may be possible to obtain voluntary compliance because the geometry of the roads in question is not generally considered desirable by truckers and because truckers may be able to comply with the restriction by moving their rear axles forward. While truck restrictions may not be possible on some or all of the National Highway System (NHS) and the designated National Network, the need for this strategy is not likely to be present on roads that are part of these systems.
Other Key Attributes
	A public education and information program will be desirable, targeted at the trucking industry.

Objective 15.2 B—Minimize the Adverse Consequences of Leaving the Roadway at a Horizontal Curve

Combined Discussion of Strategies

Objective 15.2 A focuses on helping drivers stay within the limits of the roadway while negotiating a curve. By contrast, Objective 15.2 B focuses on reducing the severity of curverelated crashes that occur outside of the roadway (i.e., on the roadside).

Despite the countermeasures (i.e., strategies) implemented in pursuit of Objective 15.2 A, some vehicles will still leave the roadway and stray onto the roadside. The strategies for Objective 15.2 B are intended to minimize the consequences to vehicles that travel beyond the shoulder and onto the roadside at a horizontal curve.

Five strategies are designed to reduce the consequences of leaving the roadway:

• Strategy 15.2 B1: Design safer slopes and ditches to prevent rollovers (P)
• Strategy 15.2 B2: Remove/relocate objects in hazardous locations (P)
• Strategy 15.2 B3: Delineate roadside objects (E)
• Strategy 15.2 B4: Add or improve roadside hardware (T)
• Strategy 15.2 B5: Improve design and application of barrier and attenuation systems (T)

These strategies are discussed in the ROR crash guide under the section entitled “Combined Strategy: Improving Roadsides,” and the reader is referred to that section for more detailed information on this set of strategies. Although these strategies are fully discussed in the ROR crash guide, it is important to at least identify them here because ROR crashes are so prevalent along horizontal curves. The reader is also referred to the Roadside Design Guide (AASHTO, 2002) for current information and operating practices relating to roadside safety.

Strategy Attributes

Vehicles that encroach upon the roadside while traversing a curve can face conditions different than if they were traveling along a tangent alignment. This issue was addressed under Strategy 15.2 A4 (Install Shoulder Rumble Strips) and Strategy 15.2 A5 (Install Centerline Rumble Strips). As a vehicle leaves the roadway, the departure angle is affected by the degree of road curvature. The concern is for the potentially more extreme condition that can occur when a vehicle leaves the road on the outside of the curve. If a vehicle drifts to the right at a constant angle along a tangent section, the vehicle can have a shallower departure angle than where the vehicle leaves the roadway on the outside of a horizontal curve. The larger departure angle is because of the curvature of the roadway. This is demonstrated, among other things, in Exhibit V-21.

This greater departure angle at horizontal curves has implications on the travel distance and time to roadside objects. Given that an object is located a certain distance from the roadway and that the vehicle follows a certain trajectory, a driver has less distance and time to regain control of his/her vehicle before striking a roadside object located at a horizontal curve than a roadside object located on a tangent.

Another way of looking at it is in terms of the change in the effectiveness of a clear zone (on the outside of the curve) of constant width. Exhibit V-21 shows that not only because of the increased angle of departure, but also because of the curvature of the clear zone adjacent to the curved segment of the road, the distance traveled by an errant vehicle, before reaching the outer edge of the clear zone, will be smaller than if the road were on a tangent alignment. Thus, the probability of an errant vehicle going beyond a constant clear zone is greater along the outside of a curve than along a tangent segment of road. This implies that roadside objects should be further removed from the roadway along the outside of horizontal curves compared with roadside objects located along tangent sections of highway.

There are several assumptions inherent in the analysis, but these are similar to the assumptions currently used in roadside safety analyses. It should also be noted that the opposite conclusion is true for encroachments on the inside of curved roads. However, clear-zone requirements are based on more than just a vehicle that drifts off the road.

The guides for addressing collisions with trees in hazardous locations and utility poles provide supplemental information on reducing the harm done by collisions after vehicles have left the roadway. Strategy 15.2 B2 in this guide directly relates to Strategy 16.1 B1 in the tree guide. Similarly, Strategy 15.2 B3 in this guide and Strategy 16.1 B4 in the tree guide are directly related. The reader is directed to these additional sections for more specific information on the effects of collisions with trees. For Strategy 15.2 B2, if the reader is seeking specific information on utility poles, then the utility pole guide should be consulted.

EXHIBIT V-21 Depiction of the Effect of Curvature on the Conditions Resulting from a Vehicle Encroaching on the Roadside

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