Many factors affect seat belt use, including sex, age and state laws, but Schlundt said the connection between increased weight and decreased use was consistent when those things were taken into account.
Government regulations for auto manufacturers don't use BMI to determine dimensions for seat belts. The standard instead says belts must fit up to a 215-pound man who has a seated hip circumference of 47 inches. That was set in the 1960s.
When the National Highway Traffic Safety Administration considered changing the rules in 2003, it estimated that more than 38 million people, or 19 percent of the total U.S. population, were larger than the seat belt requirements.
The NHTSA decided not to revise its standards since most top manufacturers including Ford Motor Co., General Motors Corp., Chrysler LLC, Nissan Motor Co. and Honda Motor Co. have seat belts that are longer than required.
The companies each provide an average of 18-20 inches of extra belt length, more than enough to accommodate the largest percentage of drivers. Many of those manufacturers also have seat belt extensions or longer belts that can be purchased or installed at dealerships. Ford offers their extensions for free, said Wes Sherwood, a Ford spokesman.
Several foreign brands, such as Honda, BMW, Hyundai, Mercedes-Benz and Porsche, do not provide seat belt extenders. Ford's Volvo division requires buyers to sign a waiver stating they've read a list of warnings and rules for using extenders, said Daniel Johnston, a Volvo spokesman.
Extensions have to be used carefully because they can be hazardous if used by passengers who are too small, said Phil Haseltine, president of the Washington, D.C.-based Automotive Coalition for Traffic Safety.
According to the NHTSA, an incorrectly sized seat belt extender could fail to provide upper body restraint and may pull the lap belt onto the abdomen during a front impact, possibly leading to internal injury.
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On the Net:
National Highway Traffic Safety Administration: http://www.nhtsa.dot.gov/