The reconstruction of SR 99 within several communities
in the Seattle area illustrates the importance of monitoring the performance
of design exception locations after construction. For
these projects, the Washington DOT established a formal in-service evaluation
agreement with the cities involved. Several
characteristics of Washington’s in-service evaluation make it a good model:
- The cooperation between the
DOT and the cities was critical for a successful data collection effort.
on crash reports alone would not have been as effective because the trees
were small during the data collection period and many impacts were not
supplementing crash data with the unreported impacts provided by city
maintenance personnel, the researchers were able to compile a much more
complete and accurate data set and gain some insights about future safety
performance as the trees mature.
- The DOT and the cities jointly
committed to implement mitigation measures, including tree removal in
some areas, if warranted by the incoming crash information.
- By monitoring performance,
some changes were made quickly, before the study was completed. For
example, when it became evident in SeaTac that many trees were being struck
at the narrow-median areas adjacent to left-turn lanes, planting in these
areas was discontinued in subsequent project phases. As
discussed in Chapter 2, some judgments on expected performance can be
made from speed studies and other driver behaviors that can be obtained
in a much shorter time frame than crash studies. Quicker,
proactive mitigation efforts are sometimes appropriate.
- The Washington DOT is using
the information learned from the in-service evaluation to evaluate its
internal urban design criteria and make modifications, if appropriate.
obtained from this evaluation is also expected to assist in decisionmaking
when similar proposals for exceptions to DOT design criteria are made
for future projects.
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