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Discussion

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. Relying 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 reported. By 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. The knowledge 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.