the past. This is the most straightforward use of a key indicator and usually implies a comparison over time, typically year-to-year changes. Second, safety indicators can indicate the status of countermeasure programs. In this case, the understanding is derived from the research underlying the development of the countermeasure in the first place. However, the meaning is likely to be limited to the context of the countermeasure. Such measures can be a useful report card for a particular program, but they may not have use beyond the program. Finally, safety indicators can be used to support policy decisions regarding existing and new countermeasures.

The final use, supporting policy decisions, may be problematic. Countermeasure development usually involves some detailed understanding of the relationship of the countermeasure to some safety objective, such as reducing injuries or accidents. Out of that understanding can come some key indicators. But time trends in key indicators characterizing the overall state of transportation safety are not likely to include any information that is relevant to decisions on new countermeasures or programs. In general, key indicators can tell in which directions things are headed, but they usually do not contain information on why the trend is up or down. Thinking that they do appears to be a common misconception with any key indicator. This theme of appropriate and inappropriate use continues through the remaining material discussing issues in the selection of key safety indicators.

Issues

Key indicators are numbers that convey information. By its nature, any indicator is necessarily an oversimplification and carries with it an inherent risk of misuse. In addition to selecting the measure, it is also necessary to document the intended meaning of the indicator. What understanding can the user expect to obtain? Four main issues were identified by the subgroup. The first issue is to identify the audience and exactly what the indicator is intended to communicate. The second issue has to do with counts versus rates. The third issue deals with data quality, and the final issue is exposure.

Purpose

Based on the group’s discussions, a fundamental issue in selecting a key indicator is to identify the audience and clarify what the indicator is in-



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