Reflections on methods of statistical inference in research on the effect of safety countermeasures. For presentation at the Annual Meeting of the Transportation Research Board, Washington, D.C., January 1983.

Sensible management of traffic safety is predicated on having reasonable expectations about the effect of various safety countermeasures. It is the role of evaluative research to derive such intelligence from empirical data. In spite of decades of research and experience, the safety effect of many countermeasures remains unknown. This sorry state of affairs is largely due to the objective difficulty of conducting conclusive experiemnts. Recognition of thi objective difficulty should lead to the realization that in transport safety, knowledge is accumulated gradually from small, noisy and diverse experiments. The statistical tools used to extract knowledge from data should reflect this aspect of reality.