A safety belt system that has won great praise for automatically strapping in drivers is drawing scrutiny from safety experts who say the belts are often misused in a way that can be dangerous or even fatal. The system consists of a motorized shoulder belt and a manual lap belt. Manufacturers have been installing them since 1984 in millions of cars. Each costs several hundred dollars less than an air bag, an alternative way to meet federal safety requirements.
When a driver or front-seat passenger closes a car door and turns the ignition, the motorized belt diagonally crosses the chest. The lap belt only works if closed manually.
A study by the University of North Carolina concludes that 70 percent of drivers and riders, lulled by the automatic shoulder strap into a false sense of security, do not fasten their lap belts.
Some safety engineers say failing to buckle a lap belt can permit an occupant, particularly one who is short, to "submarine" under the shoulder belt during a collision and strike the dashboard. Moreover, if a door flies open or the vehicle rolls, a shoulder belt may fail to protect an occupant from ejection.
Some safety experts have been concerned since 1984, when automatic shoulder straps were first put into use. Their concern has been heightened in the last two years because starting with the 1990 model year all new cars, foreign and domestic, on American roads must have a passive-restraint system _ either air bags or automatic shoulder belts.
If air bags are used, they need only be on the driver's side; if automatic shoulder belts are used, they must be for both the driver and front-seat passenger. Shoulder belts are generally designed to be used with lap belts.
There are many millions of cars in the United States with automatic shoulder restraints, and the number is increasing every day. Ford, for instance, has 3.5-million cars with motorized shoulder restraints on the road.
Safety experts are unanimous that, even when a lap belt is not buckled, a shoulder restraint is usually better than no restraint. But they are concerned that, in certain kinds of wrecks, a shoulder restraint used alone can cause injury or death.
The National Transportation Safety Board and the National Highway Traffic Safety Administration recently began studying the problem. They are combing through dozens of accident reports, but there is not yet a comprehensive list of deaths or injuries connected to the automatic-belt system.
Aware of the complaints, automakers plan to phase out the motorized belts by the mid-1990s. The companies acknowledge that the shoulder belts are not totally effective unless the lap belt is buckled. When it is not buckled, buzzers sound for about six seconds.
And warnings are posted inside the cars; a typical one includes the sentence, "Failure to wear the manual lap belt could increase the chance and/or the severity of injury in an accident."
But the warnings, often in small print, are on the top of sun visors and thus are not visible unless the visors are down.
Regulators, automakers and safety experts are certain that many people, if not a majority, do not heed the warnings. "Even dedicated seat-belt users are psychologically convinced they are buckled safely when the automatic belt crosses over them," said Clarence Ditlow, executive director of the Center for Auto Safety, a non-profit advocacy group.
Two crashes, one last year in Kentucky and the other this month in Georgia, attracted particular attention.
In the Georgia crash, a 25-year-old woman was decapitated. The victim was a front-seat passenger in a 1988 Ford Escort that had motorized automatic belts for the front seats, and was found with her lap belt unbuckled.
The car had slid across a highway near Atlanta on June 1 and was hit on the right-rear side by another automobile. The victim's husband, who had buckled his lap belt, was seriously injured. Investigators are studying whether the nylon shoulder harness that crossed his wife's neck and torso could have decapitated her.
"A nylon belt is as strong as steel; in a crash it's almost like a saw," said Dr. Joseph Burton, the medical examiner for six counties around Atlanta who performed the autopsy on the victim.
"The only logical explanation was that the belt cut through her neck and took off the head," Burton said. "We found blood on the belt, and there was no other serious injury on the body."
A Ford Motor Co. spokeswoman said Wednesday: "Though Ford's investigation is incomplete, its evaluation of the available evidence places considerable doubt as to the shoulder belt being the cause of decapitation. Ford believes any conclusion about the cause of the decapitation would be premature at this time."
When properly used _ with both the lap belt and the shoulder belt fastened _ the automatic systems generally protect users from death or serious injury, often, for example, by preventing a driver from being thrown from a car.
But without the lap belt, the protection can be greatly diminished, according to statistical studies of accidents.
Last August, for example, a 17-year-old woman from Owensboro, Ky., died of head injuries when she was thrown out of her 1989 Nissan Sentra, which had swerved and rolled over in a grass median.
William Wilson Jr., a lawyer representing the family of the woman, Jiyon Kim, said investigators noted that her motorized shoulder belt had been connected but the lap belt was found unbuckled.
A Nissan spokesman said that the automatic seat belt "exceeds federal standards" but that the lap belt must be buckled for maximum effectiveness.
And safety engineers now concur that air bags, when used with a shoulder harness and lap belt, provide the best protection.