Trees in Hazardous Locations: Appendix 13

Assessing the Benefits of the Roadside Tree

While roadside trees can, under certain circumstances, pose a hazard to errant motorists, it is imperative to assess the benefits of these trees must be assessed before making a decisiondeciding which strategy to implement to address the safety problem. This is an important part of a total assessment that an agency should perform. Completing and documenting a benefit assessment may prevent some conflicts with the agencies and citizen groups that may be focusing upon saving particular trees. Publications such as the 1979 Assessment of Guidelines for Removing Hazardous Trees from the Highway Right-of-Way1 and "Trees Make Cents"2 describe detail many of these benefits in detail. The scale of the impact is important to consider. While removal of a single tree, or a single row of trees, may have minor to negligible impacts, removing all trees along the roadside can have significant negative effects.

Environmental Impacts

Trees in forests, parks, commercial, residential areas, and in general, provide environmental benefits. However, the direct environmental benefits (not including aesthetics and historical benefits) of roadside trees to drivers and the roadway setting are minimal. Less hazardous shrubs and plants can provide many of the benefits provided by roadside trees.

Grey and Deneke found that roadside trees serve as windbreaks that could protect traffic from crosswind gusts. In areas with frequent high-wind gusts and high truck volumes, this benefit may influence the decision to remove roadside trees. Grey and Deneke also stated that trees reduce both glare and reflection. Planting shrubs may provide similar benefits without the fixed object threat. The authors further discuss other environmental benefits such as noise abatement, air filtering and wildlife habitat that may determine whether tree removal is even an option. For example, one state was not permitted to remove any trees that served as a habitat for the endangered Indiana Bat.

Roadside trees may also produce environmental hazards to consider in the assessment. If trees are too close to the road, for instance, it may create conditions, such as micro climates that influence road-surface conditions, hampering snow removal, blocking sight distance, and shielding pedestrians from the driverís view.


Blue Ridge Parkway in North Carolina


Aesthetic Value of Trees

While the aesthetic value of trees is not directly related to driving and the operation of traffic, one should consider that drivers live, work, and recreate in communities that contain the roadways. The aesthetic value of trees is a complex issue that an engineer must consider when assessing the benefits of roadside trees. The aesthetic value of trees, while difficult to measure directly, is easier to work with when using economic benefits as a surrogate measure. Economic benefits, such as higher commercial land values, increased patronage of business and spending, and higher residential property values are examples of economic benefits that serve as surrogate measures of the aesthetic value. Exhibit 13-1 shows a segment of the Blue Ridge Parkway in North Carolina and Virginia, where millions of people visit, in large part because of the trees, especially during the fall when the leaves change color. During the peak of the "leaf" season, visitors bring millions of dollars into the local economies.

A study for the Center for Urban Horticulture shows that real-estate appraisers believe that trees boost sales appeal and values; and commercial areas with trees have higher occupancy and lease rates. Residential property sales show similar trends. They found that trees increase property values an average of 5 percent as compared to an identical property without trees.4 Another study by the Center for Urban Horticulture shows customers, shopping at businesses with tree cover and extensive landscaping, spent 11 percent more than at comparable businesses without trees and landscaping. Additionally, the study found that 75 percent of consumers prefer to shop at places with trees and landscaping.5

Historical Value of Trees

Many communities developed around tree-lined roads long before the prominence of the automobile. These tree-lined streets are a part of the communitiesí identity as much as other historical features. Taking this perspective, congested streets are seen as invading the very core of the community. Removing trees, to allow wider roads, is not an option with which many citizens will agree. Using guardrail to shield motorists from the trees is generally not an aesthetically acceptable approach, given the currently available types of guardrails. In some cases, the mere mention of cutting down trees can cause a controversy. If historical tree-lined streets develop into hazardous routes, engineers and the communities need to consider other strategies to service traffic in the region.

States and counties may keep a registry of trees having historical value. The engineers responsible for investigating the hazardous sites involving trees need to consult with these registries to insure ensure that protected trees are not recommended for removal.

Evaluating Alternatives

Once a site investigation is completed, various strategies need to be identified that may reduce the frequency or severity of tree crashes at the site. The primary alternatives should always be oriented toward keeping vehicles on the road (refer to the run-off-road emphasis area guidebook for further information on this). The alternatives may be compared using incremental benefit-cost analyses, cost-effectiveness analysis, or similar methodology normally used in the respective agency.

A careful analysis should consider all benefits and costs, from a variety of perspectives. It is important, therefore, to include all stakeholders in the evaluation of alternatives. Because of the difficulty of arriving at monetary values for some of the benefits and costs, a cost-effectiveness approach may be preferable to a benefit-cost analysis, since there are techniques available for the former to include non-quantifiable elements in the assessment. In either case, it is advisable to do sensitivity analyses to test the degree to which conclusions may change, if estimated benefits and costs vary.

In addition, when comparing benefits and costs, one must consider the costs of potential conflicts that may result with those who do not want trees to be removed, or otherwise change the existing condition. Some agencies adopted a "no conflict" policy because the costs associated with these conflicts are time consuming and very expensive. However, policies of this nature may backfire if there is a "conflict" for every tree. This reiterates the importance of completing the risk and benefits assessments, and doing it with the involvement of all stakeholders.

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

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

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

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

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