Reductions in police-reported injuries associated with Michigan's safety belt law

This research measured the effects of Michigan's compulsory safety belt use law on traffic crashes and injuries of various severities. Using time-series methods, the authors analyzed monthly frequencies of crash-induced injuries and fatalities from January 1978 through December 1987. Exposure to risk of occupant injury was controlled statistically by including aggregate frequency of crashes as a covariate in time-series models. Effects of economic conditions on traffic crashes were controlled by including an index of unemployment as a covariate. The following statistically significant effects were associated with the safety belt law: (a) In crashes with minor vehicle damage, there was a 14.6% reduction in B-level injuries, an 11.0% reduction in C-level injuries, and a 13.0% reduction in aggregate (KABC) injuries; (b) in crashes with moderate vehicle damage, there was a 16.8% reduction in A-level injuries, an 11.6% reduction in B-level injuries, a 10.7% reduction in C-level injuries, and a 3.6% reduction in aggregate (KABC) injuries; (c) in crashes with severe vehicle damage, there was a 6.3% reduction in fatalities, an 11.8% reduction in B-level injuries, a 4.7% reduction in c-level injuries, and a 5.8% reduction in aggregate (KABC) injuries; (d) for all vehicle damage severities, there was a 14.0% reduction in B-level injuries, an 8.3% reduction in C-level injuries, and a 6.4% reduction in injuries to front-seat occupants. Based on these results, Michigan's adult safety belt law has prevented 31,710 injuries from July 1985 through December 1988.