5th Westminster lecture on transport safety : traffic safety measures, driver behaviour responses and surprising outcomes. Lecture given London, 6th December 1994.

This paper examines how road-user behavior changes in response to changes in traffic system safety, such as anti-locking brakes, safety belts, air bags and crash helmets, thereby generating surprising outcomes from safety interventions. In response to changes introduced to increase safety, examples are presented in which the actual outcome was that: (1) safety increased even more than expected; (2) safety increased as expected; (3) safety increased, but less than expected; (4) no observed change in safety; (5) the change actually decreased safety (a perverse effect). Correspondingly diverse reactions were found for changes expected to reduce safety, but introduced to achieve other goals.