A key component of crime deterrence is the certainty of detection, but in 2005 seat belt laws in 27 states prohibited law enforcement officers from ticketing an observed violation unless the driver is stopped for another offense, which is referred to as secondary enforcement. Thirteen states and the District of Columbia have upgraded from secondary to primary enforcement, which authorizes police to stop a motor vehicle if a driver or occupant covered by the law is observed not using a seat belt. To test the impact of seat belt enforcement provisions, cross-sectional time series regressions are estimated for annual driver and occupant fatality rates in the American states from 1990 to 2002. Using several control variables for other traffic policies and state demographics, the results indicate primary enforcement is more effective in saving lives than secondary enforcement. Furthermore, upgrading to a primary law enhances the effectiveness of an existing state mandatory use law.