States with seat belt laws that allow motor vehicle occupants to be ticketed solely for not wearing seat belts (primary) appear to have lower motor vehicle crash (MVC) fatalities rates than states with laws that allow ticketing for failure to wear seat belts in the setting of other violations (secondary). Researchers in Massachusetts found that when the number of states with primary seat belt laws increased from 16 to 30, the average MVC fatality rate per 100,000 persons decreased from 14.6 to 9.7.

Source: Annals of Internal Medicine, August 4, 2015.