If three is good, can four be better? That question is being applied to seat belts by Volvo, the pioneer of the three-point seat belt with its lap and shoulder belt.
Although the three-point seat belt is the most effective safety device we have to reduce injuries and fatalities in a crash, safety researchers at Volvo and its parent company the Ford Motor Company think they can make the three-point seat belt even safer. By making a four-point seat belt they think they can better distribute crash forces across a person's chest and hold a person in the seat more securely in a rollover, or if a car is hit from the side.
"We feel the four-point belt is the next step in the evolution of the safety belt, said Paul Bedewi, Ph.D., technical specialist, Vehicle Safety Research Department, Ford Research Laboratory, Ford Motor Company.
Volvo introduced its patented three-point belt in the front seat of cars in the Nordic market in 1959 as standard equipment and then introduced the three-point belt in the U.S. in 1963. Since then, few changes have been made to the three-point belt, but those changes include adding pre-tensioners and load-limiting retractors and making them height-adjustable.
Safety belts have saved thousands of lives since the U.S. government passed the first legislation requiring three-point seat belts in vehicles in the U.S in 1968. From 1975 through 1999, the government's National Highway Traffic Safety Administration (NHTSA) estimates this safety device has saved 123,213 lives. And while usage has reached an all-time high of 71 percent in 2000, that still leaves 29 percent of the population not wearing seat belts.
Four or three?
The questions Volvo and Ford safety engineers are asking about three-point seat belts are: Could they be made even safer for the people who wear them? Could they be more comfortable so that even more people would wear them?
The next generation of automotive safety belts using four-point belts are being studied in two versions: a V and an X shape. The V4 belt has straps that operate similar to the way the shoulder straps of a child safety seat do. The upper attachment points are located in the seatback near the shoulders. The person pulls the straps over each shoulder and fastens them to a buckle in the middle of a lap belt.
The X4 belt adds another shoulder belt to a traditional three-point belt. The conventional three-point belt is put on first by pulling it across the chest and buckling it as usual by the hip. The additional chest belt crisscrosses the chest and is buckled on the opposite side.
By adding another belt, safety researchers believe there will be two major benefits and are conducting crash tests to analyze crash performance of the four-point belt. They expect the extra belt to better hold a person in the seat if a vehicle rolls over or is hit from the side. They also expect it to distribute crash forces more uniformly over the chest and reduce the pressure by half.
"For an elderly person, redistributing those forces can reduce the number of rib fractures," said Steve W. Rouhana, Ph.D., technical specialist and group leader in Ford's Safety Research and Development Department. "At the age of 65, your ability to withstand crash forces is about one-fifth what it was when you were 20 because the chest bones deteriorate as we age."
In addition to crash tests, the companies are evaluating the comfort and convenience of the four-point belt in customer clinics to see which design people prefer and why.
Because the straps of the V design — the suspenders type — sit on the shoulder, for example, they eliminate the rubbing and chafing some people experience with the traditional three-point belt and maybe with the X design that crosses the neck on two sides.
Ford and Volvo expect to survey 5000 consumers to get statistically valid results that will translate to a four-point seat belt that will be safer and that people will wear.
"We're very confident that these systems offer real-world safety improvements but we have to make sure that people wear them more than three-point belts. The best belt system in the world is no good if you don't wear it," Rouhana said.