Safety belt use and the switch to primary enforcement, 1991-2003

State seat belt laws have increased use rates and have reduced traffic fatalities, but tremendous variation exists in the laws. New Hampshire does not have a law, and 30 states have only secondary enforcement laws. Whereas primary enforcement allows an officer to issue a citation for any infraction, secondary enforcement permits a citation only if a motorist is stopped for another infraction first. We performed a cross-sectional time-series analysis of the impact of upgrading to primary enforcement on belt use rates for 47 states and the District of Columbia from 1991 to 2003. Our results suggest that states with secondary enforcement laws could increase belt use by 10 percentage points and improve public safety considerably by upgrading to primary enforcement.