Introduction: Policies that allow directly citing motorists for seat-belt non-use (primary enforcement) have been shown to reduce motor vehicle crash deaths relative to secondary enforcement, but the evidence base is dated and does not account for recent improvements in vehicle designs and road safety. The purpose of our study was to test whether recent upgrades to primary enforcement still reduce motor vehicle crash deaths.

Methods: We used motor vehicle crash death data from the Fatal Analysis Reporting System for 2001-2014, and calculated rates using both person- and exposure-based denominators. We used a difference-in-differences design to estimate the effect of primary enforcement on death rates. We estimated negative binomial regression models controlling for age, substance use involvement, fixed state characteristics, secular trends, state median household income, and other state-level traffic safety policies (blood alcohol laws, speed limits).

Results: Models adjusted only for crash characteristics and state-level covariates models showed a protective effect of primary enforcement (Rate Ratio [RR]=0.88, 95% confidence interval [CI] 0.77 to 0.98; Rate Difference [RD]= -1.44 deaths per 100,000 population, 95%CI -2.78 to -0.11). After adjustment for fixed state characteristics and secular trends there was no evidence of an effect of upgrading from secondary to primary enforcement in the whole population (RR=0.99, 95%CI: 0.93 to 1.05; RD=-0.11, 95%CI: -1.01, 0.80) or for any age group.

Conclusion: Upgrading to primary enforcement no longer appears protective for motor vehicle crash death rates.