In a quasi-experimental study, hospital-treated traffic accident injuries were recorded prospectively for 7 1/2 years in the two Norwegian cities, Harstad and Trondheim. In Harstad the recorded data were used actively in analysis, planning, and implementation of a community-based injury prevention program. Trondheim was the nonequivalent control city. The intervention was divided into three periods, each of 30 months duration. Preventive efforts were implemented to some extent in period 1, increasingly in period 2 and period 3. Traffic safety was promoted in an extensive community program based on the Ottawa charter for health promotion. A 26.6% overall reduction of traffic injury rates was found in Harstad from period 1 to period 3 (p