Do states upgrading to primary enforcement of safety belt laws experience increased daytime and nighttime belt use?

As of 1 January 2007, 26 states and the District of Columbia have enacted primary enforcement of their safety belt laws, which allows law enforcement to stop motorists and cite them solely when they observe a vehicle occupant who is not wearing a safety belt. Interrupted time series analyses were used to determine whether six states which upgraded to primary enforcement laws experienced changes in nighttime (9:00 p.m. to 4:59 a.m.) and daytime (5:00 a.m. to 8:59 p.m.) safety belt use based on proxy estimates from fatal crash-involved vehicle occupants. Nighttime and daytime safety belt use increased in five of the six states after the primary enforcement laws were enacted. Because the methods used in these analyses reduced the likelihood that these increases resulted from preexisting secular trends towards increased belt use, the results provide strong support that upgrading from secondary to primary enforcement increases occupant safety belt use during both daytime and nighttime periods.