A new study suggests that laws allowing police to directly cite motorists for not wearing seat belts may be less effective than they once were at reducing accident deaths.

The issue: Seat belts save lives by drastically reducing the likelihood that drivers and passengers will die in a motor vehicle crash. An often-cited study published in 2003 by scholars from the National Bureau of Economic Research and Stanford University confirms that “mandatory seat belt laws unambiguously reduce traffic fatalities.” The study also estimates that for each 1 percentage point increase in the proportion of people using seat belts, 136 lives are saved annually.

Over the years, states have adopted what are referred to as “primary enforcement” or “secondary enforcement” seat belt laws. Primary-enforcement laws allow police to pull over motorists and ticket them (or their passengers) if seats belts are not in use. Secondary-enforcement laws permit officers to issue a ticket related to seat belts only if the officer already is citing someone for another traffic infraction. As of early 2017, 34 states and Washington DC have primary-enforcement laws for anyone riding in the front seat of a vehicle, according to the Governors Highway Safety Association, a group of state highway safety offices. Fifteen states have secondary laws for adults riding in front seats. New Hampshire is the only state without a seat belt law for adults.

Previous research has suggested that primary-enforcement laws improve traffic safety more than secondary-enforcement laws. A 2002 study published in Accident Analysis & Prevention, for example, demonstrated a reduction in motor vehicle injuries after California upgraded its seat belt law from secondary enforcement to primary enforcement. A 2006 study in the Journal of Policy Analysis and Management indicates that states with primary-enforcement laws have lower vehicle fatality rates than states with secondary-enforcement laws.

But Americans’ driving habits and other circumstances have changed since those studies were completed. Seat belt use alone has risen significantly – from about 71 percent nationally in 2000 to about 90 percent in 2016, according to the National Highway Traffic Safety Administration.

An academic study worth reading: “Primary Enforcement of Mandatory Seat Belt Laws and Motor Vehicle Crash Deaths,” published in the American Journal of Preventive Medicine, 2017.

Study summary: Sam Harper and Erin C. Strumpf of McGill University tested whether primary-enforcement laws still result in lower death rates among drivers and passengers in motor vehicles. The authors analyzed crash data from the Fatal Analysis Reporting System, a census for the National Highway Traffic Safety Administration, for the years 2000 to 2014. The Insurance Institute for Highway Safety provided data on seat-belt policies and the dates that some states upgraded from secondary-enforcement laws to primary-enforcement laws.

Harper and Strumpf considered, as part of their analysis, several factors they believed might influence traffic safety, including recent laws related to maximum speed limits, legal limits for blood alcohol concentration and graduated driver’s license programs.

Among the key findings:

• After controlling for certain factors, the authors find that switching from secondary enforcement to primary enforcement has almost no impact on traffic death rates. States that upgrade their laws are estimated to have 0.22 fewer deaths per 100,000 than states that keep their secondary-enforcement laws in place.
• These estimates “suggest that the impact of upgrading from secondary to primary enforcement on MVC [Motor Vehicle Crash] death rates may have waned in comparison to earlier studies.”
• The authors present several possible reasons why the impact of adopting primary-enforcement laws might have diminished in recent years. There is evidence that road improvements and changes in vehicle safety and design have contributed to lower crash death rates. The growing popularity of speed cameras and traffic roundabouts may have helped reduce deaths. Changes in the economy also may play a role as people tend to do less recreational driving when the economy contracts.

Other resources for journalists:

• The National Highway Traffic Safety Administration provides a variety of data on traffic crashes and deaths, including those involving motorcycles, pedestrians or impaired drivers.
• The U.S. Centers for Disease Control and Prevention offers state-specific fact sheets related to motor vehicle occupant deaths and seat belt use.
• The National Conference of State Legislators monitors legislation related to seat belts and child restraints.

Related research:

• A 2005 study from the Transportation Research Board, “Minorities and Primary Versus Secondary Belt Use Enforcement,” looks at how minority motorists were impacted when five states switched from secondary-enforcement laws to primary-enforcement laws for seat belts.
• A 2014 paper in the Journal of Policy Analysis and Management, “Do Traffic Tickets Reduce Motor Vehicle Accidents? Evidence from a Natural Experiment,” finds that tickets reduce car accidents and injuries.
• A 2015 study in the journal Traffic Injury Prevention, “The Effects of Vehicle Redesign on the Risk of Driver Death,” finds that “changes in vehicles, whether from government regulations and consumer testing that led to advanced safety designs or from other factors such as consumer demand for different sizes and types of vehicles, have been key contributors to the decline in U.S. motor vehicle occupant crash death rates since the mid-1990s.”
• Journalist’s Resource has featured academic research on the dangers of driving while texting and drowsy driving. We also have written about how devices that require speech activation can interfere with visual attention.