Risk of death in car crashes reduced if all occupants wear seat belts

January 20, 2004

Because an unrestrained occupant could cause harm to another occupant during an auto crash, the risk of death is lowest when all occupants are restrained by seat belts, according to a study in the January 21 issue of The Journal of the American Medical Association (JAMA).

"A car occupant could be killed if struck by another occupant who was catapulted forward, backward, or sideways in a crash," the authors provide as background information in the article. This study estimated the association between the death of a car occupant and the restraint use (seat belt of any type or a child car seat) of another occupant in the same car.

Peter Cummings, M.D., M.P.H., and Frederick P. Rivara, M.D., M.P.H., from Harborview Injury Prevention and Research Center, Seattle, analyzed data from the National Highway Traffic Safety Administration's Fatality Analysis Reporting System on U.S. traffic crashes from 1988 through 2000. The researchers restricted their study to pairs of occupants who were in the same car when they crashed.

"The study subjects who made up the cohort pairs are referred to as targets in this article to distinguish them from other occupants," the authors write. "The main study exposures for each target occupant were the presence of another occupant (not the other target pair member) who might be thrown against the target, as well as the restraint use of this other occupant. A target might be exposed to no other occupant, a restrained other occupant, or an unrestrained other occupant."

For front-seat target pairs, the main study exposure was the presence and restraint use of a rear occupant directly behind the target. For example, if there was an unrestrained right rear passenger, the target front passenger was exposed to an unrestrained other occupant. For rear targets, the exposure studied was the presence and restraint use of a front occupant directly in front of the target. And a side target pair consisted of one occupant in the front seat and one in the back seat, both on the same side of the car.

There were 61,834 cars with two front-seat targets; 7,969 targets were in front of a left rear occupant and 10,785 were in front of a right rear occupant. There were 5,278 cars with two rear targets; all left rear targets had a driver in front of them and 90 percent of right rear targets had a passenger in front of them. There were 9,648 cars with two left-side targets and 11,479 cars with two right-side targets.

"Restraint use by rear occupants might influence the risk of death of a front target if some rear occupants were catapulted forward. This mechanism is consistent with our finding that the risk of front target occupant death associated with a rear occupant was greater when the rear occupant was not restrained. ... Front occupants might be hurled against a target in the rear seat, especially when the impact is from the rear," the authors explain. "A side target may be easily hit by an occupant beside them in the same seat row; the target and the other occupant are not separated by a seat back."

The authors report that for a belted front seat occupant, the risk of death in a crash was increased by 20 percent if someone behind them was unrestrained. For a rear seat occupant, the risk of death was increased by 22 percent if someone in front of them was unrestrained. For a belted person on one side of the car, the risk of death was increased by 15 percent if an occupant in the same seat row on the other side of the car was not restrained.

In conclusion the authors write: "If the associations we have estimated are causal, use of restraints by rear seat occupants may prevent about 1 in 6 deaths of restrained front targets that would otherwise occur when a rear occupant is present. Use of restraints by front-seat occupants may similarly reduce the risk of death for all rear targets. Persons who wish to reduce their risk of death in a crash should wear a restraint and should ask others in the same car to use their restraints."
(JAMA. 2004;291:343-349. Available post-embargo at jama.com)

Editor's Note: This work was supported by grants from the Centers for Disease Control and Prevention, Atlanta, and by the Crash Injury Research and Engineering Network of the National Highway Traffic Safety Administration.

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