Trees in Hazardous Locations

Introduction

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 ROR collisions, and tree impacts are generally severe. This section addresses fatal crashes involving impacts with trees.

Tree crashes are a subset of ROR crashes. Volume 6 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 guide (Volume 3) focuses on measures to reduce 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 for ways to prevent tree crashes by keeping the vehicle on the roadway.

The Guide to Management of Roadside Trees (Zeigler, 1986) is a key resource for reducing tree-related crashes. It addresses safety versus environmental issues. In particular, highway agencies may use it to

Identify and evaluate higher risk roadsides,
Identify alternative treatments,
Identify environmental considerations regarding roadside treatment,
Provide guidelines for roadside tree removal and maintenance practices, and
Provide documentation necessary to substantiate tree removal or alternative treatments.

In addition to various references, survey information was obtained from 14 state DOTs with respect to their methods for reducing tree crashes, including how environmental issues are considered.

Appendix 2 contains the nine-question survey form and a summary of responses by state. Appendix 1 contains proposed guidelines for tree plantings and clear zones by the North Carolina DOT Roadside Environmental Unit (which may not have received final approval from North Carolina DOT for use as yet). Appendix 3 contains excerpts from the Michigan DOT Design Manual related to tree removal.

Tree hazards encompass many DOT disciplines. Tree removal, or other similar programs, 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 that

Implementing an effective program must address the concerns of construction and maintenance engineers. Guidance will be needed on roadside flattening and appropriate tree removal that may be part of a highway rehabilitation or reconstruction project.
For safety engineers, it will be necessary to describe how to identify roadway spots and sections with clusters of tree crashes to consider cost-effective tree removal safety enhancement projects.
For design engineers, it will be necessary to develop guidelines for construction of relatively flat sideslopes, clear roadside recovery areas, and landscaping plans.

One of the hallmarks of 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 in a coordinated manner. To facilitate this, the electronic guides use hypertext linkages to enable 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. 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.