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In-Service Evaluation of Major Urban Arterials with Landscaped Medians--Conditions as of 2004

Description: Arterials such as State Route (SR) 99 north and south of Seattle have characteristics that are considered by many cities to be undesirable. In response, several cities are implementing redevelopment plans to increase road safety, create a more aesthetically pleasing local environment, and enhance the economic vitality and attractiveness of the communities. The cities’ 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 the desires of cities to implement aesthetic designs, WSDOT chose to adopt an in-service evaluation process that would study collision, environmental, operational, and maintenance experiences in the field.

This report summarizes the findings of two of the 13 median treatment projects included in the in-service evaluation of alternative median treatments. It provides before data and condition information on all median sections, but because most of the roadway sections have only recently been improved, it provides after data for only two phases of improvements, those on SR 99 in the City of SeaTac. The conclusions of the before and after analysis on those two sections of roadway indicate that no dramatic changes in roadway safety have occurred as a result of the change from continuous two-way left turn lanes and limited sidewalks to a streetscape that includes full sidewalks and landscaped medians with turn pockets, and where both sidewalks and medians include small trees as part of the landscaping. While no definitive conclusions about the safety impacts of landscaped medians can be drawn at this point in the multi-year in-service evaluation, it can be said that there is no indication that the roadways are less safe than before the landscaping was planted. Therefore, it is recommended that the current in-service evaluation be continued as planned.

  • Date Published: February, 2007
  • Publication Number: WA-RD 636.1
  • Last Modified: October 8, 2008
  • Authors: Anna St. Martin, John Milton, Mark E. Hallenbeck, Jennifer Nee.
  • Originator: Washington State Transportation Center (TRAC)
  • # of Pages: 203 p., 3.29 MB (PDF)
  • Subject: Medians, Landscaping, Trees, Context sensitive design, Redevelopment, Beautification, Environmental design, Safety factors, Evaluation and assessment, State highways.
  • Keywords: Trees, highway safety, aesthetic design, Context Sensitive Design, urban design, accident rates, injury severity, SR 99.
  • Related Publications: Research Note

    In-Service Evaluation of Major Urban Arterials With Landscaped Medians--Phase II (WA-RD 636.2);
    In-Service Evaluation of Major Urban Arterials With Landscaped Medians--Phase III (WA-RD 636.3).


This abstract was last modified June 26, 2013