We examine the effectiveness of seat-belt systems for preventing mortality in automobile crashes. In particular, we examine whether seat-belt effectiveness has been constant over this period and conclude that there has been a trend toward an increased effectiveness of seat-belt systems. The relative odds of death associated with wearing seat belts dropped from 0.80 in 1975 to 0.34 in 1985. This trend results from improvements in both the type and design of seat belts and from additional design changes, such as padded dashboards and collapsible steering columns, that protect occupants at the time of collision.