U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
Monitoring the performance of design exception locations after construction is the final step in the design exception process. Because of limited financial and human resources, the extent of in-service evaluation will and should vary, but monitoring the safety and operational performance at design exception locations has several benefits. First, if problems do develop, design changes or modified mitigation techniques are warranted to improve performance. Second, the lessons learned from in-service evaluation increase the body of knowledge about the safety and operational effects of design exceptions and mitigation measures. This knowledge will lead to better decisionmaking, both in terms of evaluating design exceptions and in mitigating their potential adverse impacts.
The rare and random nature of crashes means that several years of crash data may be needed before any conclusions can be drawn as to whether a crash problem is statistically significant and whether it is related to the design exception. In addition to reviewing crash data, in-service evaluation techniques can be implemented to obtain information over much shorter time periods. Predictions can be developed from this information on how well the location will perform, and additional or modified mitigation measures can be implemented. For example, speeds can be monitored at a curve that does not meet criteria for curvature or stopping sight distance.
Advanced technologies (Figure 4) can be useful tools for collecting this type of immediate data. These technologies can also provide much more information on what is contributing to a crash problem than a written crash report, based on the limited information available at crash locations.
Case Study 3 (presented in Chapter 7) illustrates how one State is collecting and analyzing in-service data for a design that incorporated trees in a raised median. If the crash data indicate poor substantive safety, the agency has committed to removing the trees or implementing other mitigation measures.
Advanced technology is making the collection of in-service data more effective and more efficient