The risk of death increased the more obese the driver was, according to the study released Monday in the BMJ Group's Emergency Medicine Journal.
The researchers examined data from the U.S. Fatality Analysis Reporting System from 1996 to 2008. In that period, 57,491 collisions in the nation were submitted.
The study used the World Health Organization definition of obese, which is a body mass index (BMI) of 30 or higher.
The study's authors pointed to previous research that showed that an obese driver's lower body is propelled farther upon impact before a seat belt engages the pelvis. The driver's additional tissue prevents the belt from fitting snugly, but the upper body is held back.
"Research has shown that seat belts do not engage the pelvis, as it should, in the case of obese motor vehicle occupants," Thomas Rice, the study's lead author, said in an e-mail. "We would stress the importance of wearing the lap belt down low against the lap and pulled in as close to the pelvis as possible," added Rice, research epidemiologist at the Safe Transportation Research and Education Center of the University of California at Berkeley.
The authors suggest that while obese people may have underlying health problems, vehicle design may need to change to provide better protection.
"The ability of passenger vehicles to protect overweight or obese occupants may have increasingly important public health implications, given the continuing obesity epidemic in the USA," the authors said in a statement.
More than a third of U.S. adults - 35.7% - are obese, according to the Centers for Disease Control and Prevention. Some 16.9% of U.S. children and adolescents are obese.
Keshia Pollack, associate professor with the Center for Injury Research and Policy at the Johns Hopkins Bloomberg School of Public Health in Baltimore, says more testing is needed to understand the biomechanics of how bodies react in collisions. For example, there have been discussions about using larger crash-test dummies, says Pollack, who was not involved in the study.
Pollack says research also shows that obese individuals, particularly morbidly obese ones, are less likely to wear seat belts and to use them properly. More research can look at how to design seat belts to fit better and to be more comfortable.
She adds, "We also need to be focusing more on encouraging people to wear their seat belts because they are an effective technology."
The National Highway Traffic Safety Administration said it will review the report. Lynda Tran, a spokeswoman for the agency, said, "NHTSA is actively engaged in research to better understand the effects of obesity and injury risk."
(Copyright © 2013 USA TODAY)