We investigated the effect of mandatory seat belt laws on socioeconomic differences in seat belt use. We identified the differential effect of legislation across socioeconomic groups using a difference-in-difference-in-differences model based on state variations in the timing of the passage of laws. Seat belt laws consistently reduced educational and income gaps, with the smallest differences between socioeconomic groups observed among states that do not require any other violation to issue a citation for seat belt non-use (primary enforcement). Our results imply that existing socioeconomic differences in seat belt use would be further mitigated if all states upgraded to primary enforcement.