Lap and shoulder belts prevent ejection from the vehicle and keep people from colliding with the vehicle interior during a crash. They are also designed to manage forces on the body.
Even though a vehicle may have slowed or stopped after colliding with another vehicle or object, unbelted occupants keep moving at the same travel speed until they catch up with and crash into what's in front of them. Seat belts help to prevent or reduce injuries from this second collision by securing people to their seats so they slow down with the vehicle as its crush zone absorbs most of the kinetic energy associated with the vehicle and the occupant's precrash motion. The longer people “ride down” a crash, the less likely they are to be injured.
Worn properly, seat belts are designed to spread crash forces across the stronger bony parts of the body, including the shoulder, rib cage and pelvis. Seat belts also prevent occupants from being ejected from the vehicle, an event associated with high risk of injury and death. Relative to occupants who are not ejected from vehicles, occupants who are ejected in nonrollover crashes are nearly twice as likely to die, and those who are ejected in rollover crashes are 4 times more likely to die (NHTSA, 2010).
Unbelted occupants can put other people in the vehicle at risk. In a frontal crash, drivers and front-seat passengers are at increased risk of injury from unbelted back-seat passengers, and in a side-impact crash, passengers sitting adjacent to unbelted passengers are at increased risk of injury.
Exposure to unbelted occupants increases the risk of injury or death to other occupants in the vehicle by 40% (MacLennan et al., 2004). In a frontal crash, an unbelted rear-seat passenger sitting behind a belted driver increases the risk of fatality for the driver by 137% compared with a belted rear-seat passenger (Bose et al., 2013).
In both the front seat and the back seat, seat belts reduce the risk of serious injury or death in a crash. Research has shown a 45% reduction in the risk of a fatal injury to front-seat car occupants when lap and shoulder belts are used (NHTSA, 2017). The risk of a moderate to critical injury is reduced by half. For people in front seats of SUVs, vans and pickups, the use of lap and shoulder belts reduces the risk of a fatal injury by 60% and a moderate to critical injury by 65%.
In the center rear seat, lap and shoulder belts reduce the risk of fatal injury by 58% in cars and 75% in SUVs, vans, and pickups (Kahane, 2017).
The National Highway Traffic Safety Administration (NHTSA) estimates that seat belts saved 20,443 lives in 2019 and a total of 457,578 lives between 1968 and 2019 (Kahane & Simons, 2024).
What if a seat belt doesn’t fit or has only a lap belt?
For seat belts to do their job, it’s important that they fit right. That’s why it is so critical for children who have outgrown their forward-facing child restraints to use belt-positioning boosters.
A properly fitted shoulder belt should lie snugly across the center of the chest and shoulder, not on the neck or face or falling off the shoulder. The lap belt should lie snugly across the upper thighs or low on the hips, not the belly.
Pregnant drivers and passengers should place the lap belt low on the hips and under the belly. It’s important to continue using a seat belt during pregnancy; doing so reduces fatal injury risk for a pregnant woman and her fetus (Klinich et al., 2008).
When shopping for a new vehicle, try on the seat belts to see whether they are a good fit for you and the other passengers who will be riding in the vehicle. In many vehicles, belts can be customized for a better fit. For example, adjustable D-rings allow shoulder belts to be raised or lowered as needed. Ask your dealer or consult the vehicle manual for more information. If your seat belt is not long enough, it may be possible to lengthen it with a belt extender from your vehicle manufacturer.
A properly fitting belt offers the best protection, but any restraint is better than none at all.
Similarly, a lap belt alone is better protection than no belt in a crash. Today’s vehicles have three-point belts in all seating positions, but vehicle built before September 2007 may only have lap belts in the rear center seats (NHTSA, 2004).
In a study of potentially fatal crashes involving back-seat occupants age 5 and older, lap belts reduced the risk of fatal injury for outboard occupants by 32% in cars and 63% in vans and SUVs (Morgan, 1999). Although lap-only belts weren’t as effective as lap and shoulder belts, particularly in frontal crashes, using lap-only belts provided more protection than being unrestrained.