Trees in Hazardous Locations
One of the most common causes of fatal and severe injury crashes, on rural roads in particular, involves vehicles leaving the road and striking a fixed object. Trees are the objects most commonly struck in run-off-road (ROR) collisions, and tree impacts are generally quite severe. This section addresses crashes involving impacts with trees.
Tree crashes are a subset of ROR crashes. Emphasis Area 15.1 addresses the general subject of ROR crashes. It covers strategies aimed at reducing the consequences of ROR crashes by keeping vehicles from leaving the roadway and reducing the severity of impacts after leaving the roadway. This volume focuses on measures directed at reducing the harm in tree crashes after encroachment on the roadside has occurred, such as removing trees and shielding motorists from trees. The reader should refer to Volume 6 of this report for strategies aimed at preventing tree crashes by keeping the vehicle on the roadway.
One of the key resources for guidance on reducing tree-related crashes is the Guide to Management of Roadside Trees (Zeigler, 1986). The guide addresses safety versus environmental issues. In particular, highway agencies may use it to
Information from the Guide to Management of Roadside Trees was considered while developing this document. In addition, survey information was obtained from 14 state departments of transportation (DOTs) with respect to their methods for reducing tree crashes, including how environmental issues are considered.
The issue of tree hazards encompasses many DOT disciplines. Tree removal or other similar programs must address planning, design, construction, and maintenance as they relate to roadway and roadside features that affect tree crashes. The literature and experience of DOTs suggest the following:
One of the hallmarks of the American Association of State Highway and Transportation Officials' (AASHTO's) Strategic Highway Safety Plan is to comprehensively approach safety problems. The range of strategies available in the guides will cover various aspects of the road user, the highway, the vehicle, the environment, and the management system. The guides strongly encourage the user to develop a program to tackle a particular emphasis area from each perspective. To facilitate this, hypertext linkages are provided in the electronic version of this document (see http://safety.transportation.org) to allow seamless integration of various approaches to a given problem. As more guides are developed for other emphasis areas, the extent and usefulness of this form of implementation will become more apparent.
The goal is to move away from independent activities of engineers, law enforcement, educators, judges, and other highway-safety specialists. The implementation process outlined in the guides promotes forming working groups and alliances that represent all of the elements of the safety system. In so doing, the groups can use their combined expertise to reach the bottom-line goal of targeted reduction of crashes and fatalities associated with a particular emphasis area.
General Description of the Problem
Collisions between vehicles and trees are a major type of traffic fatality. According to Fatal Accident Reporting System data for 1999 (Exhibit I-1), 10,967 fatal crashes involved a fixed object. Trees were the objects most often struck, involving 3,010 fatal crashes, or about 8 percent of all fatal crashes. Exhibit I-2 shows the distribution of fatal crashes by roadway functional class. Fatal tree crashes were most prevalent on local rural roads, followed by major rural collectors. Of all fatal tree crashes, 90 percent occurred on two-lane roads and 5 percent on four-lane roads (Exhibit I-2). While reducing tree-related fatal crashes will require addressing all classes of streets and highways in urban and rural areas, rural two-lane roads will receive much of the focus in the development of any program to reduce tree-related highway fatalities (Exhibit I-3).
Objectives of the Emphasis Area
The goal of this emphasis area is to eliminate tree crashes or reduce the harm that results from colliding with a tree. A primary way of accomplishing this is to keep the vehicle on the road. The strategies for this area are covered in Volume 6 of NCHRP Report 500. The objectives for this emphasis area are
While the occupants of the vehicle that leaves the roadway and potentially strikes a tree are the ultimate concern of this objective, the direct focus is on the roadside environment. These objectives are targeted at larger trees, generally greater than 4 in. in diameter. Smaller trees, like small wooden sign supports, typically break away or bend over and are less likely to result in serious consequences. Exhibit 1-4 summarizes the objectives and related strategies presented in this guide.EXHIBIT I-1
Total and Fixed-Object Fatal Tree Crashes in 1999
1999 Fatal Tree Crashes by Functional Class
Fatal Tree Crashes by Number of Travel Lanes, 1999
Emphasis Area 16.1Crashes with Trees in Hazardous Locations
Trees contribute significantly to the roadway environment. This can be a complicating factor in dealing with trees in hazardous locations. There is a strong movement nationally to maintain and preserve historic and scenic resources during construction and reconstruction of highways. Strategies that focus solely on the safety aspects of trees and promote tree removal over other measures will not be acceptable to important constituencies. Many states have developed extensive scenic byway programs. The concept of context-sensitive design has been adopted in much of the country, and it is endorsed by AASHTO. It encourages a comprehensive view of the design situation, in a collaborative framework.