Summary and Closing Notes

In summary, Example Organizational Objective #1 has illustrated how to use the model TIM performance measurement database to assess overall performance (i.e., average ICT for all freeway incidents). This example has also illustrated how to use the database to assess TIM performance at a more disaggregate level (i.e., average/median ICT for freeway incidents by incident severity and injury severity), as well as the benefits of doing so.

A closing note on the use of the “incident severity” data element – while the classification of incidents based on severity can be subjective, a section entitled, TIM Performance by Incident or Injury Severity of the guidance outlines the classification/definition of incident severity according to the MUTCD. The MUTCD classifies incidents as “major,” “intermediate,” or “minor” based on incident duration and other incident characteristics including the types and number of vehicles involved, the number of lanes involved, and whether traffic control is needed. In the model TIM performance database presented in this guidance, incident severity is also classified as “major,” intermediate,” or “minor.” When selecting an incident severity category, agencies may want to refer to the MUTCD classification/definition, or they may have their own way of classifying incident severity. Because the MUTCD provides roadway clearance times in the classification/definition of incident severity, incident severity could be classified after calculating the RCT performance measure for each incident rather than at the time of the incident (as is done in Arizona). According to the MUTCD, incidents with road closures lasting less than 30 minutes can be considered “minor” incidents; incidents affecting travel lanes for 30 minutes to 2 hours can be considered “intermediate” incidents; and incidents that affect all or part of the roadway for more than 2 hours can be considered “major” incidents. In this case, a subroutine can be built into the TIM database to populate the incident severity field based on RCT or ICT, rather than entering it at the time of the incident.

Organization Objective #1 has also illustrated the application of performance targets, and as such, there is one closing note and consideration for agencies on setting performance targets. While setting and using targets is an important part of performance measurement and management, agencies should be careful not to allow their targets to drive actual performance. An example is shown in the figure below.

The graph shown in the figure below illustrates how internal policies related to incident clearance could impact performance. A,C, D and E represent four different actual transportation agencies. Agencies A, C, and E did not set specific performance targets; rather their philosophy was to clear the roadway as quickly as possible. As a result, their clearance profiles show that the majority of their incidents were cleared from the lanes in 15 minutes or less. Agency D, however, had a performance target to clear 90 percent of all incidents within 90 minutes. From the graph it can be seen that Agency D has a completely different clearance time profile than the other agencies. Due to the shape of the clearance profile, it appears that Agency D’s policy provided little incentive for the agency to clear its roads significantly less than the target of 90 minutes. The 90-minute target of agency D seems to have placed focus on the 90 minutes rather than a goal of clearing incidents as quickly as possible.

Lane Clearance TimeDistribution of Lane Clearance Times for Four Transportation Agencies

(Explanation and graphic courtesy of Michael Pack. Used with permission.)