Seat belt legislation and effectiveness evidence

The engineering case for seat belts had been established for decades before any American state required their use. Crash biomechanics research from the 1950s onward had documented that restrained occupants sustained far fewer severe injuries than unrestrained ones in equivalent collisions. Through the 1970s, efforts to get Americans to buckle up relied mainly on awareness and education campaigns and achieved limited success.

New York enacted the country's first seat belt use law in 1984, and it was fully in force, with civil fines of $50, from January 1, 1985. Among drivers in crashes, reported belt use rose from 21 percent before the law to 47 percent after it, and a roadside survey in and around New York City found 63 percent use, against 16 percent before. That study, published in the Journal of Trauma in 1986, compared three months before the law with five months after it; among front-seat passengers use rose from 21 to 40 percent, too few cases to be statistically significant. The authors urged physicians to seek similar laws in every state.

John States and colleagues at the University of Rochester compared 18 months of crash admissions in Monroe County before the law with 18 months after it, linking police crash reports to hospital records. Belt use among hospitalized occupants rose from 11.2 percent to 53 percent, admissions fell 11.9 percent while rising 2.6 percent among motorcyclists, cyclists and pedestrians, and the average injury severity score for occupants fell from 16.01 to 14.55. The study was observational, and spine and abdominal injuries rose after the law even as other injuries fell. Seeing how much more effective the law was than education campaigns, safety advocates turned to passing laws in other states.

Enforcement varies by state: as of September 2023, 29 states did not have a primary enforcement law covering all adult seating positions. Primary enforcement laws, which allow officers to stop vehicles solely for belt non-compliance, produced higher observed belt use than secondary laws requiring another violation as a prerequisite. In 2022 observed belt use was 92 percent in states with primary enforcement and 90 percent in states with secondary laws or none.

According to the CDC, seat belts saved almost 15,000 lives in the United States in 2017. In the 1980s Mothers Against Drunk Driving, founded in 1980, helped pass nearly 250 state drunk driving laws in little more than a decade. William Haddon Jr., the first administrator of the National Highway Traffic Safety Administration, had earlier pushed road safety toward a public health approach, including his matrix of factors before, during and after a crash.