Several cities have implemented redevelopment plans that include the re-design of major regional arterials in order to raise the quality of life of those living, working, and shopping along or near the arterial. Many of these redevelopment efforts include landscaped medians with trees placed close to the roadway including in the median. WSDOT’s clear zone width criterion may not always be met when trees are placed within medians. To address this potential conflict, an in-service evaluation process was adopted to study collision, environmental, operational, and maintenance experiences in the field.
This report updates earlier work published in 2007 and 2009. It examines before and after periods for all 13 roadway sections identified by WSDOT for review. Some of these 13 sections contain trees in medians with no protection, others are sections that contain raised medians but no trees, and others are control sections.
The study finds that the presence of small trees in the median did not significantly increase crash rates, crash severity, or injury crash rates. Crash rates decreased at statistically significant levels for both types of median treatment locations. No significant difference was found when comparing median treatments with and without trees. At test sites, crash rates remain stable six years after the treatments were completed, indicating that the safety benefits first observed remain in place over time. It appears that adding small trees to landscaped medians does not have a detrimental effect on safety. Installation of medians and access control as part of a more general increase in access control generally result in a decrease in midblock crashes, but an increase in crashes occurring at intersections where turning movements are allowed, in large part because turns are concentrated at those locations. These increases are a fraction of the midblock gains, resulting in improved safety overall.
Washington State Transportation Center (TRAC)
Medians, Landscaping, Trees, Redevelopment, Arterial highways, Beautification, Environmental design, Safety factors, Accident rates, Evaluation and assessment.