Trees in Hazardous Locations

Description of Strategies

Objective

Once a problem with tree crashes has been identified, detailed analyses are needed at each site. Site investigations to determine potentially effective strategies contain two parts: (1) risk assessment and (2) benefits assessment. Risk assessment helps determine the relative risk of future tree crashes. Benefits assessment determines the local effects of removing trees or the application of other strategies. It may help for an agency to develop a structured approach to assess the risk and impacts. A proactive approach to preventing tree crashes involves use of policies and guidelines to prevent or remove unsafe tree growth. Various strategies are available to help achieve the objectives listed below.

The objectives for reducing the number of head-on fatality tree crashes are

16.1 A—Prevent Trees from Growing in Hazardous Locations and
16.1 B—Eliminate the Hazardous Condition and/or Reduce the Severity of the Crash.

The objectives and the applicable strategies are tabulated below in Exhibit V-1 and described in detail in the following tables.

EXHIBIT V-1
Emphasis Area 16.1—Crashes with Trees in Hazardous Locations

Objectives	Strategies
16.1 A—Prevent Trees from Growing in Hazardous Locations	16.1 A1—Develop, Revise, and Implement Planting Guidelines to Prevent Placing Trees in Hazardous Locations (T)^a 16.1 A2—Mowing and Vegetation Control Guidelines (P)
16.1 B—Eliminate the Hazardous Condition and/or Reduce the Severity of the Crash	16.1 B1—Remove Trees in Hazardous Locations (P) 16.1 B2—Shield Motorists from Striking Trees (P) 16.1 B3—Modify Roadside Clear Zone in the Vicinity of Trees (P) 16.1 B4—Delineate Trees in Hazardous Locations (E)
^a For an explanation of (T), (E), and (P), see next page.

Types of Strategies

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 state or even local levels. Some have been subjected to well-designed evaluations to prove their effectiveness. On the other hand, 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 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:

Tried (T)—Those strategies that have been implemented in a number of locations and may even be accepted as standards or standard approaches, but for which valid evaluations have not been found. These strategies, while in frequent or even general use, should be applied with caution, carefully considering the attributes cited in the guide and relating them to the specific conditions for which they are being considered. Implementation can proceed with some degree of assurance that there is not likely to be a negative impact on safety and very likely to be a positive one. As the experiences of implementation of these strategies continues under the AASHTO Strategic Highway Safety Plan initiative, appropriate evaluations will be conducted, so that effective 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 proven not to be appropriate or feasible. Even where considered, their implementation should initially occur using a very controlled and limited pilot study that includes a properly designed evaluation component. Only after careful testing and evaluations show the strategy to be effective should broader implementation be considered. 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.
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 understanding that any application can lead to results that vary significantly from those found in previous evaluations. The attributes of the strategies that are provided will help the user judge which strategy is the most appropriate for the particular situation.

Related Strategies for Creating a Truly Comprehensive Approach

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

Public Information and Education Programs (PI&E)—Many highway safety programs can be effectively enhanced with a properly designed PI&E campaign. The primary experience with PI&E campaigns in highway safety is to reach an audience across an entire jurisdiction or a significant part of it. However, a PI&E campaign can be focused 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 for, or Relation to, Support Services"). In some cases, the strategy is explained in detail for PI&E campaigns deemed unique for the emphasis area. As additional guides are completed for the AASHTO plan, they may detail PI&E strategy design and implementation. When that occurs, the appropriate links will be added from this emphasis area guide.
Enforcement of Traffic Laws—Well-designed and well-operated law-enforcement programs can have a significant effect on highway safety. It is well established, for instance, that an effective way to reduce crashes and their severity is to have jurisdictionwide programs that enforce an effective law against driving under the influence (DUI) or driving without seat belts. When that law is vigorously enforced, with well-trained officers, the frequency and severity of highway crashes can be significantly reduced. This should be an important element in any comprehensive highway safety program. Enforcement programs, by nature, are conducted at specific locations. The effect (e.g., lower speeds, greater use of seat belts, and reduced impaired driving) may occur at or near the specific location where the enforcement is applied. This can often be enhanced by coordinating the effort with an appropriate PI&E program. However, in many cases (e.g., speeding and seat-belt usage) the impact is areawide or jurisdictionwide. The effect can be either positive (i.e., the desired reductions occur over a greater part of the system), or negative (i.e., the problem moves to another location as road users move to new routes where enforcement is not applied). Where it is not clear how the enforcement effort may affect 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, it is often in support of some other strategy. Many of those strategies may be targeted at either a whole system or a specific location. In such cases, the description for that strategy will suggest this possibility (see the attribute area for each strategy entitled "Associated Needs for, or Relation to, Support Services"). 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 detail the design and implementation of enforcement strategies. When that occurs, the appropriate links will be added from this emphasis area guide.
Strategies to Improve Emergency Medical and Trauma System Services—Treatment of injured parties at highway crashes can significantly affect the level of severity and length of time an individual spends in treatment. This is especially true when it comes to timely and appropriate treatment of severely injured persons. Thus, a basic part of a highway safety infrastructure is a well-based and comprehensive emergency care program. While the types of strategies included here are often thought of as simply support services, they can be critical to the success of a comprehensive highway safety program. Therefore, for this emphasis area, it should be determined if improvements can be made to this aspect of the system, especially for programs focused 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 detail the design and implementation of emergency medical systems strategies. When that occurs, the appropriate links will be added from this emphasis area guide.
Strategies Directed at Improving the Safety Management System—The management of the highway safety system is vital to success. A sound organizational structure should be in place, as well as infrastructure of laws, policies, etc., to monitor, control, direct and administer a comprehensive approach to highway safety. A comprehensive program should not be limited to one jurisdiction, such as a state DOT. Local agencies often have most of the road system and its related safety problems to deal with. They also know, better than others, what the problems are. As additional guides are completed for the AASHTO plan, they may detail 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—One very logical manner for preventing crashes with trees alongside the road is to keep the vehicles on the road and in their proper travel lane—i.e., prevent ROR crashes. This objective is Goal 15 of the AASHTO Strategic Highway Safety Plan. The 1999 FARS data show that of 37,043 fatal crashes, nearly 39 percent were single-vehicle ROR crashes. Volume 6 of this report has been developed for addressing run-off-road crashes. It is strongly recommended that the set of strategies in Volume 6 be considered in combination with the strategies in this guide.

Any program targeted at the safety problem covered in this emphasis area should be created having given due consideration to the inclusion of other applicable strategies covered in the following guides:

-Volume 4, Head-On Crashes
-Volume 5, Unsignalized Intersections

Objective 16.1 A—Prevent Trees from Growing in Hazardous Locations

This objective subscribes to the adage that prevention is better than the cure. This is true in many circumstances with roadside trees. The approach is a proactive one. While trees provide many benefits, they can cause hazardous situations if they are too close to the road. Not only can trees develop into a fixed-object hazard, but they can also block important signs, decrease sight distance at intersections and curves, and obstruct drivers' vision of pedestrians and other roadway users. This objective is not intended to prevent the planting and growing of roadside trees. Instead, it encourages the DOT, communities, and conservation groups to develop planting and maintenance guidelines where trees and roadways can safely co-exist to meet reasonable safety, aesthetics, and operational standards.

EXHIBIT V-2
A Tree Located in an Obviously Hazardous Location
Although resilient, this tree shows the scars of vehicle strikes on both sides of the tree facing traffic.

This objective is intended to prevent trees from growing in hazardous locations. This sounds simple and straight to the point, but there is more to it than that. Some hazardous locations are easily recognizable, as exemplified by the resilient tree in Exhibit V-2, but others are not as simple as this. Questions such as, "What defines a hazardous location?" or "How big a tree is too big?" require a significant amount of work with safety engineers, landscape architects, community representatives, and conservation groups before they can be answered. Simply developing a "one size fits all" guideline does not address the important issues of all stakeholders. For instance, requiring a 30-foot clear zone in all cases is not reasonable, as pointed out by the AASHTO Roadside Design Guide (1996). As with many controls used in highway design, the planting and maintenance guidelines should also consider vehicle speed, roadway curvature, purpose of the roadway, and type of facility users.

The public expects higher standards on higher-classified facilities. This expectation includes the areas of safety, operations, and aesthetics, each of which is being increasingly considered equal in producing a design. The three areas are not mutually exclusive and can coexist without detracting from each other. In addition, legal liability issues will come into play.

See Appendix 12 for details involved in identifying and addressing hazardous locations.

Strategy 16.1 A1—Develop, Revise, and Implement Planting Guidelines to Prevent Placing Trees in Hazardous Locations (T)

General Description

This strategy involves developing guidelines for placing trees along streets and highways during new construction, widening, re-landscaping, and other projects. It pertains to placing trees along the roadside or in the median of divided facilities. During the planning and design phases of highway projects, transportation officials, engineers, and community representatives have the greatest opportunity to meet the needs and desires of all stakeholders. An agency should develop planting guidelines to help protect future projects from developing into hazardous situations after the roadside trees mature and to avoid environmental and community issues that may be encountered when mitigating the hazards of trees in dangerous roadside locations. (See Exhibit V-3.)

Application Opportunities

There are many opportunities to apply the products of this strategy (the planting guidelines) throughout the normal operations of a transportation agency, including designing and constructing new facilities. Construction of new facilities does not occur as frequently as roadway widening and lower-cost projects. Consideration should also be given to smaller projects of this type as candidates for applying planting guidelines. For further details, see Appendix 5.

Components of a Planting Guideline

Planting guidelines should define the boundaries for planting areas along new and existing facilities. The operations and purpose of the road should have a bearing on the placement of trees and other plants next to the roadway. Some states have defined minimum distances from the traveled way at which plants may be placed, as a function of speed limit. While there are several components that planting plans must specifically address, the most important two for safety are the offset from the road and the conditions that affect the offset. The conditions that affect the offset are road curvature; tree size; design speed (operating speed for existing facilities); and steepness of the sideslope. Other important issues include defining tree size (what determines "large" and "small" trees); tree species; overhead environment; and who is responsible for approving planting plans, as well as permitting exceptions to the guidelines. For further details, see Appendix 6.

EXHIBIT V-3
Strategy Attributes for Developing, Revising, and Implementing Planting Guidelines to Prevent Placing Trees in Hazardous Locations (T)

*Technical*
Target(s)	This strategy indirectly targets errant motorists and motor vehicles that leave the traveled way and are at risk of striking trees along the roadside or in the median of a divided roadway.
Expected Effectiveness	Policies often take considerable time after implementation before it is possible to develop sufficient data to analyze impact. The effectiveness of the strategy is dependent upon three main factors: the guidelines, the level of implementation, and the frequency of exceptions. The guidelines must address wide-ranging issues such as lateral displacement of encroaching vehicles, the purpose of the roadway, community values, environmental issues, and other safety concerns. If the guidelines are too weak, they will be ineffective. If the guidelines are too strict, it will be difficult to gain support. The quality of the guidelines also affects other factors that influence strategy effectiveness. The level of implementation has an effect on the success. If the guidelines are implemented sparingly, the effectiveness will more than likely be negligible. If the guidelines are applied only to major arterials, most of the roadways will not be covered, and the effectiveness of the strategy will be limited. It is practically impossible to develop guidelines that accommodate every situation. Therefore, there will be exceptions. If there are frequent exceptions, this is a sign of inadequate guidelines. A large number of exceptions creates holes that reduce the effectiveness of the strategy and encourages other exceptions.
Keys to Success	A key to the success of this strategy is implementing a comprehensive planting guideline that meets the safety needs of the facilities, is environmentally acceptable, and is sensitive to community values. Inclusiveness is part of the key to success. When developing planting guidelines, the agency must include representatives from landscape architecture, environmental and scenic organizations, traffic safety engineering, roadside maintenance engineering, and other stakeholders with interest in highway safety, roadside environment, and community aesthetics. While jointly developing and adopting a guideline with the participation of all stakeholders is important, it may not be sufficient unless upper management of the responsible agency is in agreement with it.
Potential Difficulties	Many states and local DOTs may already have planting guidelines, but could overlook this strategy. While it may be true that planting guidelines exist, the extent of their application, effectiveness, and thoroughness should be reviewed to determine if tree crashes are declining both in frequency and severity. A guide may exist that does not meet the current needs. A review of the origins of the existing guidelines, and who the participating organizations were, can help determine if the planting guidelines are meeting the needs of current stakeholders. The frequency with which challenges, exceptions, and general disagreement occurs among the stakeholders is an indication of possible need to review the guidelines. In many jurisdictions, the agencies responsible for the design, approval, and maintenance of the roadside may not be familiar with the safety implications of the roadside environment. These agencies are often responsible for the environmental and aesthetic concerns of the roadside, which while important, should not conflict with the safety of the motorist. Landscaping plans should require levels of scrutiny similar to those applied to other areas of highway design dealing with fixed objects along the roadway.
Appropriate Measures and Data	The single most important process measure is the ultimate existence of a planting guide. This may include rating it in terms of adequacy of elements addressed, as well as the degree to which it meets current understanding of safety considerations. The number and rate of exceptions to the guidelines are other process measures. A thorough guideline will have few exceptions. Finally, the number and percent of projects meeting guidelines can be used to indicate how well the guides are being applied. Appropriate performance measures for this strategy include the number and rate of tree crashes, including severity. However, the initial effect may be small because most guidelines are not applied retroactively to previously implemented planting projects. Crash and roadway/roadside data (focusing upon tree crashes) are key items needed to provide the information for evaluation work.
Associated Needs	A brochure may be needed to inform organizations such as the local chapters of the Institute for Transportation Engineers, American Society of Landscape Architects, Scenic America, and other interested organizations of the changes being implemented regarding roadside-planting guidelines.
*Organizational and Institutional*
Organizational, Institutional, and Policy Issues	State DOTs and many local agencies have the organizational structures to implement this strategy. On new construction and changes to the existing roadway, engineers responsible for the inspection and acceptance of the work would also ensure that the guidelines are applied. Stakeholders from other organizations need representation when developing or revising guidelines. These organizations and institutions exist at various levels with different organizational structures. The agency responsible for developing and implementing the guidelines should actively recruit participation from landscape architects, environmental and scenic organizations, and other community representatives.
Issues Affecting Implementation Time	Developing consensus among the various stakeholders may take time; however, participants at the National Tree Symposium held in 2000, who represented a variety of disciplines, indicated that there is a desire to develop guidelines that reflect the values of engineering, environmental, and community organizations. The development, revision, and adoption process may vary from one jurisdiction to another. A thorough review, discussion, and resolution may take several months. Depending upon the practices of each agency, it may take more time to adopt the revised plan and to train the individuals responsible for applying the policy.
Costs Involved	The key cost component is personnel time to develop, review, and revise the planting guidelines.
Training and Other Personnel Needs	Training currently provided to design engineers, construction engineers, and those persons responsible for reviewing and approving plans must cover planting guidelines, along with other highway design topics. Emphasis must be placed upon the connection between highway safety and the guidelines that have been established.
Personnel Needs	No additional personnel should be required for implementation; however, some personnel resources will be required to develop, revise, and disseminate the guidelines.
Legislative Needs	None.
Other Key Attributes	None.

Strategy 16.1 A2—Develop, Revise, and Implement Mowing and Vegetation Control Guidelines (P)

Introduction

This strategy involves developing guidelines for maintaining the roadside in a way that prevents the natural growth of trees in hazardous locations or prevents trees developing into other hazards such as sight obstructions or overhead hazards. Many of the same concepts described in the planting guideline strategy (16.1 A1) apply here. The major difference is that the target trees in this strategy are not purposefully planted (often referred to as "volunteers"). Since this aspect of planting management is often dealt with by different parts of the DOT, the material is presented separately from general planting guidelines. However, where similarities exist, reference is made to the coverage in the planting guideline strategy (16.1 A1).

This strategy has been proven to reduce the severity and frequency of a variety of crashes such as fixed object crashes and crashes caused by obstructed sight distance. However, this strategy targets trees that grow naturally in hazardous locations. Trees are living organisms that reproduce at rates that vary according to the tree species and roadside environment. The main purposes of the mowing and vegetation guidelines are to achieve consistency and to provide guidance to field personnel responsible for maintaining the roadside.

The application of this strategy is feasible to all road types. Many state and local agencies have mowing and vegetation control plans, and much has been written on the subject (e.g., see http://transportation1.org/safetyplan/pubs_progs/strbrowse.asp?ele=hws&goa= 16&str=46). The guides are developed to make sure that signs, guardrails, other traffic control devices, and safety appurtenances are visible and effective. Routine maintenance also helps maintain sight distance at intersections and horizontal curves. Regular mowing (even close to safety appurtenances) cuts saplings when they are small, before they grow into a roadside hazard. Removing the trees while they are small also diminishes the temptation to save the trees if they were left to mature in a hazardous location near the roadside. (See Exhibit V-4.)

EXHIBIT V-4
Strategy Attributes for Developing, Revising, and Implementing Mowing and Vegetation Control Guidelines to Prevent the Natural Growth of Trees in Hazardous Locations (P)

*Technical*
Target(s)	By preventing trees from growing in hazardous locations, this strategy indirectly targets errant motorists and motor vehicles that leave the travel lanes and are at risk of striking trees along the roadside or in the median of a divided facility.
Expected Effectiveness	Refer to the Planting Guidelines Strategy Attributes section on "expected effectiveness."
Keys to Success	Refer to the Planting Guidelines Strategy Attributes section on "keys to success."
Potential Difficulties	Refer to the Planting Guidelines Strategy Attributes section on "potential difficulties."
Appropriate Measures and Data	Refer to the Planting Guidelines Strategy Attributes section on "appropriate measures and data."
Associated Needs	Refer to the Planting Guidelines Strategy Attributes section on "associated needs."
*Organizational and Institutional*
Organizational, Institutional, and Policy Issues	Agencies should consider including in the mowing contracts removing small trees from around roadside objects like signs and mailboxes. Uprooting trees while they are very young requires little physical effort and does not damage the roadside.
Issues Affecting Implementation Time	Refer to the Planting Guidelines Strategy Attributes section on "issues affecting implementation time."
Costs Involved	Refer to the Planting Guidelines Strategy Attributes section on "cost involved."
Training and Other Personnel Needs	Refer to the Planting Guidelines Strategy Attributes section on "training and other personnel needs."
Legislative Needs	None.
Other Key Attributes	None.

Objective 16.1 B—Eliminate the Hazardous Condition and/or Reduce the Severity of the Crash

This actually addresses two objectives, because an agency will consider each simultaneously when investigating and treating high-crash locations. In most cases, engineers decide on a treatment after an investigation has been conducted. It is preferable to keep the vehicles on the road in the proper lane. Emphasis area 15.1 deals with these in the context of ROR crashes. This section describes strategies that lessen the severity of crashes after a vehicle enters the shoulder or median.

Removing tree(s) in the clear zone, shielding vehicles from tree(s) with attenuation devices, or improving the clear zone can lessen the severity of a crash. While these strategies are not new and not only applicable to tree crashes, they can improve roadside safety. These strategies are typically responsive, rather than proactive. However, an agency may choose to develop a proactive program using one or more of these strategies as the desired treatment.

Strategy 16.1 B1—Remove Trees in Hazardous Locations (P)

Tree removal involves identifying and removing trees along highways having a history of tree crashes and/or removing trees with a high likelihood of being struck. The application of this strategy is primarily a responsive approach to treat high tree-crash locations. However, this strategy can be applied proactively to locations with high likelihood of developing tree-crash histories. Identifying the sites requiring treatment, in both responsive and proactive applications, is one of the major challenges. Another considerable, and often more difficult, challenge is overcoming public resistance to removing trees. For further discussion on assessing the value of trees, see Appendix 13.

Once the decision is made to selectively remove trees, there are other safety considerations. Not only is tree removal dangerous to the individuals performing the task, but it is also important to make sure that the remaining roadside is left in a safe condition. Large stumps, stumps on the sideslopes, and deep depressions are hazards that may remain after removing the tree. (See Exhibit V-5.) For further details on stump removal, see Appendix 9.

EXHIBIT V-5
Strategy Attributes for Removing Trees in Hazardous Locations (P)

*Technical*
Target(s)	(Responsive) This strategy directly targets locations where errant motorist and motor vehicles leave the traveled way and strike a tree in the median or on the roadside. (Proactive) This strategy targets locations that have a high likelihood of a vehicle leaving the roadway and striking a tree in the median or on the roadside.
Expected Effectiveness	The effectiveness of this strategy depends on how comprehensive the program has been in providing a sufficient clear zone on each section of highway. However, there is little doubt that a well-targeted program that provides an ample clear zone can significantly reduce tree impacts. The Pennsylvania DOT has developed a table (see Appendix 8) for estimating crash reduction for situations where numerous trees are present, relating the reduction to the distance of the tree line from the traveled way.
Keys to Success	(Responsive) Having a data analysis methodology and supporting database that can enable an agency to easily identify roadway spots and sections that experience tree crashes is one key to success. (Proactive) Developing a set of parameters that define locations with a high likelihood of tree crashes occurring is the first key to success when applying this strategy in a proactive manner. Another key is identifying the locations either through a database that contains quantitative information concerning the parameters noted for both responsive and proactive approaches or through a safety audit program. The third key is to implement a countermeasure at the locations to prevent or reduce the severity of future crashes. Involvement of all stakeholders, from the beginning of the effort, is important to optimize the results and avoid costly and contentious resistance to plans. Any tree removal program needs to target a substantial sample of road sections each year to have any effect on overall problems. Limiting the program to too few locations will not have a noticeable impact for a long time and may erode confidence in the program.
Potential Difficulties	Preservation of trees as part of the roadway's scenic character is a hallmark of the rapidly growing context-sensitive design movement. Citizens and environmental groups often strongly oppose the removal of trees within highway rights-of-way, even if there has been a long history of tree-related motor vehicle crashes, deaths, and injuries. There are also specific cases where certain trees cannot be removed, such as trees along historic routes. An objective assessment of the risks and benefits of the removal of trees in hazardous locations is essential when mitigating these circumstances. A thorough investigation can limit the impacts of treating the potentially hazardous locations. Care must be taken that tree removal be accompanied by other appropriate actions, such as removal of other hazardous roadside objects. Not completing these activities may not reduce the hazard to the driver, but merely transfer the hazard, resulting in a no net safety gain. Finally, the great amount of relatively low-volume highway mileage and prevalence of trees suggest that it will be difficult for any agency to identify substantial numbers of locations with a demonstrated tree hazard. Reliance on analysis of historic crash data for particular locations as the basis for a tree removal program may not be an effective approach. Systemic or proactive approaches may be necessary. Exhibit V-6 shows a tree-lined residential road with relatively low-traffic volumes and speeds; and having no pavement markings for the edge or centerlines. Even though the trees are relatively close to the road, they should not be targets of a tree-safety program, unless there is a demonstrated safety problem. For situations like these, if engineers attempt to establish larger clear zones along the entire length of the street, confrontations may result that will lead conservationists and community leaders to resist removal of every tree, even the ones with documented tree-crash problems. In general, care must be taken to choose the appropriate places to remove trees, focusing upon locations where problems are most severe.
Appropriate Measures and Data	Due to the improvements, performance measures may include the reduction in the number and percent of tree crashes, as well as in the number of related deaths and injuries. Process measures may include number of trees removed, number of sites treated, total expenditure for tree removal, and some measure of net change in clear zone width or area. To determine the effectiveness of the strategy, appropriate comparisons are needed of periods before and after the improvements are made. (Responsive) A listing of sites and sections with a high incidence of tree-related crashes should be produced. If possible, this listing should account for different roadway types, area type (urban or rural), etc. The top 50 or 100 sites within each highway district could be identified, so some areas of the state are not left out of the process. After identifying high tree-crash sites, supplemental information should be obtained for each site, such as traffic volume, sideslope, tree locations, and characteristics of the roadside. Many of the data elements needed require either a well-developed roadway inventory system, or special reconnaissance of selected sites.
Associated Needs	To be effective, this strategy does not require educating the public to drive differently. However, it will most likely be desirable to work with local environmental groups and to educate the public concerning the benefits of the program before implementing tree removal activities. It may be advantageous to develop materials describing the relative hazards of trees and describing how DOTs will deal with trees in a context-sensitive environment.
*Organizational and Institutional*
Organizational, Institutional, and Policy Issues	States and local DOTs have the organizational structures to implement this strategy. District offices or contractors hired by the DOT often perform this work. A key here is to develop the systems to encourage a cooperative planning effort involving the highway agency, environmental agency, and private groups.
Issues Affecting Implementation Time	Lack of data relating to tree-related crashes and their locations could adversely affect the implementation times, especially if special data collection is required. In addition, if any trees are considered environmentally significant, historically significant, or not located on the right-of-way, the process needed to determine if it is possible to remove a tree may require significant time. These factors may require use of alternative strategies to mitigate a safety problem. After the decision to proceed with a program, the process moves relatively quickly if there is little resistance and the trees are within the right-of-way. It is possible to have a tree-removal program implemented within a year.
Costs Involved	Costs will vary due to a variety of factors. Most highway agencies have considerable experience with tree removal, so generalized cost estimates are not needed here.
Training and Other Personnel Needs	Ideally, experienced and trained tree removal crews will carry out the actual tree removal and no additional training will be required for this task. (Responsive) If DOTs do not have personnel experienced in identifying hazardous locations, some training will be necessary to identify the locations at which to implement the strategy.
Legislative Needs	Legislatures in some states are involved in tree-related issues. For example, the North Carolina State Legislature recently passed a bill that encourages planting trees next to highways for beautification purposes. State DOTs must consider such legislation when establishing guidelines for tree removal. For example, such a bill might lead to plans for replacing cut-down trees either with trees that would never grow to more than 4 in. in diameter or by replanting new trees in other, less hazardous locations.
Other Key Attributes	Completing and documenting the risk and benefit assessment conducted at the investigated sites should be an integral part of implementing this strategy. Consideration should be given to developing a standard investigation form for all hazardous tree sites to standardize the approach. This information will be valuable when working with citizen groups and other individuals when considering tree removal as an option.

Strategy 16.1 B2—Providing Guardrail to Shield Motorists from Striking Trees (P)

EXHIBIT V-6
A Tree-Lined Residential Road

This strategy involves installing guardrail beyond the edge of the roadway to reduce the risk of motorists running into trees. When applying this strategy, keep in mind that guardrail is reported to be the fourth most frequently struck fixed object for fatal crashes in the United States. Another area to consider, when applying this strategy, is the additional costs and risks associated with guardrail end treatments. These costs and risk typically make this a responsive strategy. However, there are no technical reasons for not applying the strategy as a proactive approach.

This strategy is proven to reduce the severity of fixed-object crashes. The expected effectiveness depends upon a wide variety of factors, and the reader should consult the AASHTO Roadside Design Guide (1996) and associated software to determine the level of effectiveness for the specific conditions. See Appendix 7 for additional illustration and Appendix 10 for further considerations.

The placement of a guardrail or median barrier may increase the frequency of crashes in the treated area, but should lower crash severity. A 1999 study by Hunter et al. found that installing a median barrier on a freeway section significantly increased the frequency of crashes. The barrier also contributed to decreases in the number of fatal and severe injury crashes and the severity index. While in this case the purpose of the barrier was to prevent across-median crashes, one expects similar trends (increase in total crashes and decrease in severity) with the installation of barriers for other purposes.

Most in-service barriers have few environmental impacts, but the construction of the barriers may affect nearby environmentally sensitive areas. The placement of the barriers may also interfere with snow removal and restrict sight distance, especially in curves and at intersections, when placed close to the traveled way.

Increased maintenance and the difficulty in mowing around the barriers need consideration as well. The cost of repairs and maintenance will vary with the type of guardrail used, but these are often not negligible. Some agencies attempt to collect repair costs from the drivers, or their insurance companies, who strike the barriers. For example, one state collects roughly $2 million annually to help cover the expense of guardrail repairs due to motor vehicle crashes. While the collected amount is considerably less than the actual expense incurred by the DOT to repair the damaged guardrail, it is still a significant part of it. (See Exhibit V-7.)

EXHIBIT V-7
Strategy Attributes for Providing Guardrails to Shield Motorists from Striking Trees (P)

*Technical*
Target(s)	This strategy targets the errant motorist and motor vehicle that leaves the travel lane and is at risk of striking a tree along the roadside. The design of the barrier either redirects or cushions the vehicle, providing a less severe impact.
Expected Effectiveness	Guardrail installations in front of trees will typically reduce crash severity of ROR crashes, although crash frequency may increase in some cases, since a rigid object is placed closer to the roadway than are the trees or other objects being shielded. Guardrail installation may be particularly useful to reduce severity of crashes at sites with long, steep (e.g., 3:1 or steeper) sideslopes, since vehicles on steep slopes are likely to travel to the bottom of the slope (and tree removal would not prevent severe vehicle rollovers). It would also be useful in environmentally sensitive areas where trees may not be removed. With so many factors influencing the crash severity and the change in crash frequency, it is difficult to provide a simple table for expected effectiveness. Agencies are referred to the FHWA computer program ROADSIDE 5.0, or Appendix A of the Roadside Design Guide (1996) to complete economic analyses of the existing and proposed conditions (installing guardrail). The economic analysis is necessary to determine if the benefits of placing the barrier outweigh the disadvantages.
Keys to Success	A key to success is developing an effective process to identify trees in hazardous locations and to establish an effective set of criteria on when and how to install guardrails in these conditions instead of removing trees or implementing other strategies.
Potential Difficulties	A major pitfall exists when sites are improperly chosen for guardrail application. The difficulty will arise from expending resources in the name of safety, but experiencing either no net safety gain or a net degradation in safety, given the presence of the guardrail as a fixed object. Guardrail designs may not exist that meet the aesthetic requirements of those involved in the effort. Therefore, it may become necessary to develop special rails to address context-sensitive design issues on a project. These would have to meet existing design and test requirements (e.g., NCHRP Report 350, Ross et al. [1993]).
Appropriate Measures and Data	Impact measures can include the change in frequency, percent, and severity of ROR and fixed-object crashes, by type. Process measures include the number of feet/miles of guardrail installed, expenditures on new guardrail, and maintenance and repair expenditures. Good crash and roadway data are needed regarding sites where tree crashes are a problem, along with follow-up information on roadside/roadway characteristics to decide whether guardrail installation is appropriate. Good data are also required on the safety performance of guardrail and end sections for ranges of speed and roadway conditions.
Associated Needs	It may be advantageous to develop materials explaining the relative safety merits of guardrail versus tree removal, or untreated conditions, for use by planners and designers working on environmentally sensitive projects.
*Organizational and Institutional*
Organizational, Institutional, and Policy Issues	State and local DOTs are responsible for guardrail installation. However, it is important to involve all stakeholders in the process of identifying where problems exist, as well as identifying and selecting strategies to implement.
Issues Affecting Implementation Time	Lack of data and analytical tools to identify locations with high concentrations of tree crashes could affect implementation time. Where a project involves environmentally sensitive areas, the need for involving a range of groups to agree on a course of action may cause significant delays.
Costs Involved	Costs of guardrail installation and maintenance will vary significantly by the type used. Most highway agencies have adequate experience for their jurisdiction, so no specific numbers need be provided here.
Training and Other Personnel Needs	No additional training required. Trained DOT personnel or contractors are needed to correctly install, repair, and maintain the guardrail. Highway agencies must be sure that their staff is kept up to date on developments in this area.
Legislative Needs	None identified.
Other Key Attributes	Well-planned policies and guidelines are needed in the DOT on when, where, and how to install guardrails.

Strategy 16.1 B3—Modify Roadside Clear Zone in the Vicinity of Trees (P)

This strategy involves any change to the sideslope or roadside clear zone designed to reduce the likelihood of tree crashes by increasing the chances that an ROR vehicle can successfully recover without striking a tree. While both tree removal and shielding strategies modify the roadside, this strategy may be implemented in a variety of ways, such as flattening or grading sideslopes, regrading ditch sections, adding shoulder improvements, or providing protective plantings on the roadside. The cost to modify the roadside is often considerably higher than tree removal and guardrail installation. However, applying this strategy on specific curves or short tangent sections of roadway may help manage the costs. Further discussion may be found in Appendix 11.

This strategy has been proven to reduce the severity of ROR crashes and rollover crashes. While no specific studies were identified that related to only trees, much work has been completed on the benefits of improving the geometry of the roadside to allow vehicles to recover when they encroach the roadside. (See Exhibit V-8.)

EXHIBIT V-8
Strategy Attributes for Modifying Roadside Clear Zone in the Vicinity of Trees (P)

*Technical*
Target(s)	This strategy targets the errant motorist and motor vehicle that leave the traveled way and are at risk of striking a tree along the roadside. The principle of the strategy is to provide a larger recovery area in locations with a high frequency of tree collisions.
Expected Effectiveness	Effectiveness depends upon the type of improvement. For example, flatter sideslopes reduce the probability of rollover and fixed-object collisions. The following table provides information on crash reductions that may be expected due to improvements that flatten various sideslopes (based on Zegeer et al., 1987).
Keys to Success	The DOTs' willingness, and the availability of resources, to spend adequate funds to improve the roadside are important, because the job must be done completely and properly if the desired impact is to be attained. A partial effort within a project will not be sufficient. It is important that estimates of effectiveness of the proposed treatment be clearly defined in terms of differences in both frequency and severity of impact with the tree(s). Otherwise, unrealistic expectations may be created.
Potential Difficulties	This strategy may be feasible only on roads in flat to mildly rolling terrain. Significant roadside improvements in mountainous terrain are typically impractical due to cost and constructibility.
Appropriate Measures and Data	Process measures include the number of linear feet (or miles) of road where improvements are made (by type), a measure of change in clear-zone width or area, and expenditures on improvement, by type. One might also calculate the number of trees no longer in a hazardous location. The impact measures are the frequency, percent, and severity of tree crashes, ROR crashes, and total crashes. Reliable data are needed on locations of tree crashes and on the characteristics of the roadsides at the most hazardous sites. These are needed to determine where (and how) this strategy should be applied, as well as to conduct performance analyses.
Associated Needs	None.
*Organizational and Institutional*
Organizational, Institutional, and Policy Issues	A carefully crafted policy/practice is needed within the DOT on the types of roadsides that warrant specific roadside improvements and how those improvements are to be designed.
Issues Affecting Implementation Time	The time required to implement this strategy will vary, depending upon the specific type of improvement, extent of treatment to the roadside, and other factors. For projects that can be developed within the right-of-way and that do not involve roadway realignment, the period can be relatively short on the order of 1 to 2 years. For projects that require right-of-way acquisition or major construction, a longer time 2 to 5 years may be typical. In environmentally sensitive areas, the projects may require environmental permits, which can add significant delays.
Cost Involved	The cost of the project depends upon the extent and type of roadside improvement. Most highway agencies have sufficient history with these types of improvements to quickly estimate costs.
Training and Other Personnel Needs	Training is needed in both problem identification and proper installation methods. Both types of training are applicable to many aspects of DOT operations and already exist. Typically, DOT maintenance personnel or contractors would complete these improvements.
Legislative Needs	None identified.
Other Key Attributes	Applying this strategy to projects already planned can reduce the cost of improvements of this type since a significant portion of smaller projects contain a "mobilization" cost, which is difficult to quantify.

EXHIBIT V-9
Expected Percent Crash Reduction from Sideslope Flattening

Sideslope in Before Condition	Sideslope in After Condition
	3:1	4:1	5:1	6:1	7:1 or Flatter
2:1	2	10	15	21	27
3:1	0	8	14	19	26
4:1		0	6	12	19
5:1			0	6	14
6:1				0	8

Strategy 16.1 B4—Delineate Trees in Hazardous Locations (E)

This strategy follows the hierarchy as presented in the AASHTO Roadside Design Guide (1996):

Remove the object or obstacle (Strategy 16.1 B1);
Redesign the roadway, object, or obstacle so it can be safely traversed (partially included in Strategy 16.1 B3 and covered proactively in Strategies 16.1 A1 and 16.1 A2);
Relocate the object (covered in a proactive manner in Strategies 16.1 A1 and 16.1 A2);
Reduce the impact severity;
Shield drivers from the object (Strategy 16.1 B2); and
Delineate the obstacle if the above alternatives are not appropriate.

EXHIBIT V-10
Reflective Band on a Tree

Notice that the last bullet indicates that delineating the object only if the other alternatives are NOT appropriate. This strategy should not be considered a substitute for other, more appropriate strategies just because it is very low cost. The effectiveness of this strategy is not known and is currently under study in Pennsylvania. Pennsylvania places a 4-in. band of reflective tale around trunk. (See Exhibit V-10 and Appendix 14.)

The Manual for Uniform Traffic Control Devices (MUTCD; FHWA, 2000) provides guidance on the height of object markers. The guidance states:

When used for marking objects in the roadway or objects that are 2.4 m (8 ft) or less from the shoulder or curb, the mounting height to the bottom of the object marker should be at least 1.2 m (4 ft) above the surface of the nearest traffic lane.
When used to mark objects more than 2.4 m (8 ft) from the shoulder or curb, the mounting height to the bottom of the object marker should be at least 1.2 m (4 ft) above the ground.

Please note that the 4-in. band does not meet the Type 2 standard requirements in the MUTCD, the minimum width of the MUTCD standard is 6 in. wide with a minimum 12 in. in length.

There are no valid evaluations of this strategy, primarily because it is relatively untried. It is currently being experimented in several counties in Pennsylvania.

Key References

“Estimating Tree Diameter.” University of Minnesota Extension Service Website. February 21, 2001.

"Special Study — Motor Vehicle Collision with Trees Along Highway, Roads and Streets: An Assessment." National Transportation Safety Board. Washington D.C. May 1981.

Measuring Standing Trees Determining Diameter, Merchantable Height, and Volume." Ohio State University Extension Website. February 21, 2001.

American Association of State Highway and Transportation Officials. Roadside Design Guide. Washington, D.C. January 1996.

Asplundh Environmental Services. Assessment of Guidelines for Removing Hazardous Trees from Highway Rights-of-Way. Michigan Department of Transportation. Lansing, Michigan. November 1979.

Brabec, E. "Trees Make Cents." Scenic America Technical Information Series. Washington, D.C. Volume 1, Number 1, 1992.

California Department of Transportation. Highway Design Manual. Sacramento, California. Chapter 900 Landscape Architecture. July 1995.

Federal Highway Administration. Manual on Uniform Traffic Control Devices for Streets and Highways — Millennium Edition (Incorporating: Revision No. 1 dated December 28, 2001 Errata No. 1 dated June 14, 2001). Washington, D.C. December 2000.

Federal Highway Administration. Vegetation Control for Safety A Guide for Street and Highway Maintenance Personnel. FHWA 90-003 Washington, D.C. 1990.

Fitzpatrick, J. F., M. N. Sohn, T. E. Silfen, and R. H. Wood. The Law and Roadside Hazards. Michie Company. For the Insurance Institute for Highway Safety. Charlottesville, Virginia. 1974.

Grey, G. and Deneke, F. Urban Forestry. John Wiley and Sons, New York, New York. 1978.

Hunter, W. W., J. R. Stewart, K. A. Krull, H. F. Huang, F. M. Council, and D. L. Harket. In-Service Crash Evaluation of Three-Strand Cable Median Barrier in North Carolina, Report for the North Carolina Governor's Highway Safety Program, Chapel Hill, North Carolina. University of North Carolina Highway Safety Research Center, Raleigh, North Carolina. September 1999.

O'Day, J. Identification of Sites with a High Risk of Run-Off-Road Accidents. UM-HSRI-79-39. University of Michigan, Highway Safety Research Institute. Ann Arbor, Michigan. 1979.

Ross, H. E., Jr., D. L. Sicking, R. A. Zimmer, and J. D. Michie. NCHRP Report 350: Recommended Procedures for the Safety Performance Evaluation of Highway Features. Transportation Research Board of the National Academies, Washington, D.C., 1993.

The Traffic Safety Toolbox: A Primer on Traffic Safety. Institute of Transportation Engineers. Washington, D.C. 1993. Page 150.

Trees in Business Districts: Positive Effects on Consumer Behavior! University of Washington, Center for Urban Horticulture. Seattle, Washington. 1999.

Urban Forest Values: Economic Benefits of Trees in Cities. University of Washington, Center for Urban Horticulture. Seattle, Washington. 1999.

Washington Department of Transportation. Design Manual. Chapter 700 Roadside Safety. Olympia, Washington. April 1998.

Zegeer, C. V., J. Hummer, D. Reinfurt, L. Herf, and W. Hunter. Safety Effects of Cross-Section Design for Two-Lane Roads. FHWA. October 1987.

Zegeer, C., R. Stewart, D. Reinfurt, F. Council, T. Neuman, E. Hamilton, T. Miller, and W. Hunter. "Cost-Effective Geometric Improvements for Safety Upgrading of Horizontal Curves." Federal Highway Administration. Washington D.C. October 1990.

Zeigler, A. J., Guide to Management of Roadside Trees. Federal Highway Administration. Washington, D.C. December 1986.