Tree Planting and Clearing Guidance with Consideration of Minimized Crash Risk

Improvement projects are increasingly introducing landscape elements to add aesthetic appeal to the projects. Though effective for improving aesthetics, the introduction of trees on the roadside may increase the risk of fatal or incapacitating crashes. In general, the goal of roadside design is to minimize, in so far as is practical, the chance of fatal or incapacitating injury crashes on the roadside. It may not be possible to minimize that risk to the level implied in AASHTO’s Roadside Design Guide while capturing the benefits provided by trees, but it is still desirable to understand the risk presented by the presence of trees and balance that risk with the aesthetic benefits. The widely adopted benefit–cost methods currently used in the Roadside Design Guide present a significant challenge with respect to the consideration of trees. Many purported benefits of trees have not been or cannot be quantified in dollars; this lack makes the traditional unit of measurement (i.e., dollars) in a benefit–cost analysis unavailable. This paper presents a quantitative approach for assessing the risk of fatal and incapacitating injuries presented by various tree spacing and offsets. This approach can be applied to any roadway where tree planting or removal is being considered, to quantify the risk of the current and proposed tree locations such that informed decisions can be made about the risk introduced by trees and whether the risks outweigh the benefits.