Neck sprains and strains, commonly known as whiplash, are the most frequently reported injuries in U.S. insurance claims. In 2007, the cost of claims in which neck pain was the most serious injury was about $8.8 billion, or 25 percent of the total payout for crash injuries.

Head restraints help prevent whiplash. When a vehicle is struck from the rear, the seatback pushes against an occupant's torso and propels it forward. If the head is unsupported, it lags behind the torso until the neck reaches its limit, and the head suddenly whips forward. A good head restraint prevents this by moving an occupant's head forward with the body during a rear-end crash.

Head restraints should be properly adjusted. The top of the head restraint should be even with the top of the head or, if it won't reach, as high as it will go. The distance from the back of the head to the restraint should be as small as possible.

IIHS head restraint ratings

The Institute evaluates vehicles for rear crash protection by conducting sled tests with a special dummy that has a realistic spine.