The study presents empirical evidence of crash risk during and following rain events in the cities of Calgary and Edmonton, Canada. The matched sample approach is used to examine data for 169 rain events and nearly 15,000 collisions that occurred during the years 1979-1983. The overall collision risk during rainfall conditions was found to be 75% higher than normal, with travel risk being especially high during the first few hours of a rain event. The data suggest that although road pavement frequently remains wet for one hour or more after the rain has stopped collision risk returns to normal immediately.