The objective of the study was to examine the effect of a community-based injury prevention program on traffic injuries. A population-based quasi-experimental design was used with pre- and post-implementation measurements in an intervention and a control area. The program was based on inter-organizational participation in detecting and taking action against traffic injuries. The total relative risk for traffic injury in the study area showed only a tendency to decrease following program exposure (odds ratio 0.91; 95% confidence interval 0.81-1.02). No change in relative risk was observed in the control area. The analyses of program impact on injury severity showed that the relative risk for moderate injuries in the study area was reduced by almost half (odds ratio 0.59; 95% confidence interval 0.49-0.69), the risk for severe or fatal injuries remained constant (odds ratio 1.27; 95% confidence interval 0.80-2.02), and the risk for minor injuries increased (odds ratio 1.34; 95% confidence interval 1.13-1.59). The relative risk for moderate injuries was reduced by at least half for mopedists, cyclists, pedestrians, and those leaving or entering a motor vehicle. Community-based injury prevention can be a complement to national traffic safety programs.