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