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Status Report, Vol. 37, No. 9 | October 26, 2002 Subscribe

Not your father's head restraintNew designs reduce neck injuries

Head restraints have been required in cars since 1969, but until recently most of the restraints weren't high enough or close enough to the backs of many occupants' heads to provide effective protection against neck injury in rear-end crashes. This is changing rapidly. Auto manufacturers are improving the designs of head restraints, partly in response to the Institute's consumer ratings of restraint geometry by vehicle make and model (see "Head restraints: Geometry improves, and some have advanced designs," Oct. 6, 2001). Some manufacturers are doing more, adding features designed to reduce neck injury risk dynamically during crashes.

Nationwide, Progressive, and State Farm insurance companies have supplied data for a new study of the improved seats and head restraints. Institute researchers analyzed the data, finding that many of the designs are benefiting occupants in rear-end crashes. Neck injuries are being reduced.

The potential of the dynamically responding seat/head restraint designs introduced by Volvo and Saab was demonstrated in crash tests conducted a few years ago (see "Status Report special issue: neck injuries in rear-end crashes," May 22, 1999). The new study is the first in the United States to measure the effectiveness of the redesigned components in real-world crashes.

The Institute studied three approaches used to redesign seats and head restraints:

  • A straightforward approach is to improve the geometry of the restraints so they can be positioned behind and closer to the backs of most occupants' heads. This way, the restraints can protect more people in crashes. A number of manufacturers have made such improvements. Among them is Ford, which improved the geometry of the restraints in 2000-02 Ford Taurus and Mercury Sable models compared with earlier models of the same cars.
  • Saab introduced an active head restraint. As an occupant's torso sinks back into the seat during a rear-end crash, a mechanism in the seatback pushes the restraint up and toward the back of the head. Besides Saabs, some General Motors and Nissan models are equipped with these restraints.
  • Volvo and Toyota focused on seatbacks, designing them to yield in rear-end crashes to reduce the forward acceleration of occupants' torsos. Volvo dubs its design a whiplash injury prevention system (WHIPS), which includes a specially designed hinge at the bottom of the seatback allowing it to move rearward to reduce forward torso acceleration. This system includes head restraints with good geometry — that is, high and close to the back of the head. Toyota's seat design, which the automaker calls a whiplash injury lessening (WIL) system, allows an occupant's body to sink farther into the seatback during a rear impact.

Three approaches, same goal

These approaches are different, but they reflect the common goal of reducing the differential motion of an occupant's head and torso in a rear-end crash. Unsupported, the head will lag behind as the torso is accelerated when a car is hit from behind. This differential motion will cause the neck to bend backward in a motion that resembles the lashing of a whip — the higher the torso acceleration, the more sudden the motion.

The key to reducing whiplash injury risk in rear-end crashes is to keep the head and torso moving together. The redesigned seats and head restraints are all intended to do this by reducing the differential motion. The question addressed in the Institute's new research is whether these redesigned seats and head restraints are doing a better job of reducing neck injuries in real-world crashes, compared with older designs.

1999 Ford Taurus head restraint

Poor: 1999 Taurus

2000 Ford Taurus head restraint

Much better: 2000 Taurus

The geometry of the head restraints in many cars, including the 1999 Ford Taurus, is inadequate. In a rear-end crash, the unsupported head lags behind as the torso is accelerated forward. This differential motion can lead to whiplash injuries. To reduce such injuries, some automakers are introducing new seat and/or head restraint designs.

Ford improved the head restraints in 2000 Taurus models, positioning them higher and closer to the head so they're in position to reduce neck injuries. Ford also installed locks to make sure adjusted head restraints stay adjusted.

"There's evidence they're doing a better job. In some cases the reductions in insurance claims for neck injuries are dramatic," says Institute chief operating officer Adrian Lund, an author of the research report.

The researchers identified 2,641 property damage liability claims for rear-end crashes of the cars included in the study — Taurus and Sable models with and without improved restraint geometry, Volvo S70s with and without WHIPS, Toyota and Lexus models with and without the WIL system, plus a number of Buick, Nissan, Pontiac, and Saab models with and without active head restraints. The rates of insurance claims for driver neck injuries in the rear-end crashes were compared before and after the seat and head restraint design changes. Then the comparisons were adjusted to account for differences in crash severity and driver gender.

Only cars with similar rear-end structures before and after the manufacturers introduced the new seat/head restraint designs were included in the study. If a car underwent substantial rear-end structural changes at the same time, it was excluded. Thus, the changes in neck injury claim rates revealed by the research can be attributed to the seat and head restraint design changes instead of to differences in how the cars' structures managed the forces of the rear impacts.

Comparative neck injury reductions

A main finding is a 43 percent reduction in neck injury claim rates for the Saab, General Motors, and Nissan models with active head restraints, compared with similar cars before such restraints were introduced.

"The movement of this active restraint in a rear-end crash is very small, a matter of inches. However, the small movement makes a very big difference in terms of reducing the differential motion of the head and neck. A 43 percent reduction in neck injury claims is a huge improvement," Lund points out.

Similar before/after comparisons of Volvos and Fords also yielded reductions in claim rates — a 49 percent reduction in the Volvos with WHIPs, compared with earlier models without WHIPS, and an 18 percent reduction for the Fords with improved restraint geometry. However, these results weren't definitive because they weren't statistically significant.

Findings for Toyota's WIL system aren't as good. Neck injury claim rates didn't decrease. "There was too little data on the Toyota system, and the data were inconsistent across the insurers. But what we do know from this study is that the WIL system isn't reducing the injuries," Lund explains.

New generation of seat/head restraint designs:
Percent change in neck injury claim rates for new designs compared with old designs

Saab's active restraint -43%
Ford's improved restraint geometry -18%
Volvo's WHIP -49%
Toyota's WIL system +15%

Gender differences

Where reductions in neck injury claim rates were found, they were greater for women than for men. Saab's active head restraint design produced a 55 percent reduction in claim rates for women, compared with a 31 percent reduction for men. The effects of Ford's improved restraint geometry were a 37 percent claims reduction for women compared with a non-significant 8 percent increase for men. Volvo's WHIPS was associated with a non-significant 69 percent reduction in neck injury claim rates for women, while the change for men was negligible.

Even after the women in the study reaped virtually all of the benefits of the innovative designs, their neck injury claim rates weren't lower than the rates for men.

"This isn't surprising because the women's rates were so much higher to begin with," Lund says. "Throughout whiplash injury research, the finding has been that women are at greater risk of neck injury, so it's good that they seem to be enjoying more of the benefits of the improved designs."

Dynamic testing is on the way

The next step in evaluating the new restraint designs is to figure out what it is about each design that makes it perform better or worse than the others. To do this, the Institute will conduct dynamic tests of the seat/head restraint combinations. In the controlled conditions of the tests, the researchers hope to determine the specific differences in performance.

"Of course, we won't ever be able to prevent every whiplash injury claim from being filed, no matter how effective the restraint designs become," Lund also says. One reason is that some of the whiplash claims involve exaggeration of the symptoms or even fraud.

Still, the steps manufacturers are taking "do look promising. They can reduce the instance of bona fide neck injuries," Lund concludes. "This is important because the injuries can be painful, and the symptoms can last a long time. Whiplash injuries also can be expensive to treat, so reducing them would be welcome from a financial as well as a public health perspective."

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