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In-Service Evaluation of Major Urban Arterials With Landscaped Medians-Phase II

Description: Because of undesirable characteristics of some major arterials such as State Route (SR) 99 north and south of Seattle several cities are implementing redevelopment plans. These redevelopment proposals for SR 99 and other state routes include landscaped medians, many with trees placed close to the roadway in either the median or shoulder areas. However, WSDOT’s clear zone width criterion may not always be met when trees are placed within curbed medians. To address this potential conflict, WSDOT chose to adopt an in-service evaluation process that would study collision, environmental, operational, and maintenance experiences in the field.

A report was prepared in 2007 that summarized an analysis of two of the 13 median treatment projects. That report also provided 'before' data and condition information on all median sections. This report summarizes the analysis of five additional sections. Because of changes in the methodology, previously reported accident rates were recalculated to be consistent with the 'after' data from this analysis. Data from the before and after periods were also analyzed at two control locations where no median treatments were installed.

Total, fatal, fixed object, pedestrian/bicycle, tree, curb/median and U-turn accident rates were analyzed. The percentages of intersection-related, driveway-related, and other accidents were analyzed. Overall, accidents decreased significantly for the treatment locations, but no other types of accidents showed significant changes at either the treatment or the control locations, except for the percentage of other types of accidents at the control locations, which increased significantly.
It appears that the installation of landscaped medians does not have a detrimental effect on safety and may result in an overall decrease in accidents, conclusions consistent with those of the first phase.

  • Date Published: July, 2009
  • Publication Number: WA-RD 636.2
  • Last Modified: August 19, 2009
  • Authors: Peter M. Briglia Jr., Zachary N. Howard, Emily Fishkin, Mark E. Hallenbeck, Anna St. Martin.
  • Originator: Washington State Transportation Center (TRAC)
  • # of Pages: 52 p., 607.90 KB (PDF)
  • Subject: Medians, Landscaping, Trees, Context sensitive design, Redevelopment, Beautification, Environmental design, Safety factors, Accident analysis, Evaluation and assessment, State highways.
  • Keywords: Trees, highway safety, aesthetic design, Context Sensitive Design, urban design, accident rates, injury severity, SR 99.
  • Related Publications: In-Service Evaluation of Major Urban Arterials With Landscaped Medians--Conditions as of 2004 (WA-RD 636.1);
    In-Service Evaluation of Major Urban Arterials With Landscaped Medians--Phase III (WA-RD 636.3).


This abstract was last modified June 26, 2013