This study evaluated three states that changed to primary enforcement of safety belt laws, collecting statewide seat belt observation data for several years and conducting monthly seat belt observations for 6 months after passage of the primary law in 6 California cities, 5 Louisiana cities, and 5 Georgia cities (average 1,000–6,000 vehicles a month);​ monthly motorist surveys at DMVs for 6 months in these sites (n=7,061);​ focus groups with law enforcement officers and supervisors in each site;​ and citations issued locally and statewide for several years.

Observed belt use increased substantially in California (+18 percentage points) and in Louisiana (+16 percentage points). Smaller increases were seen in Georgia initially, but grew the following year. In each state, most motorists understood that they could be stopped by a police officer for a belt use violation alone. In Louisiana and Georgia, there was no evidence that one racial group was targeted for enforcement as compared to any other racial group.