A rear-end crash occurs every 17 seconds in the United States. But a surprising number of vehicles offer inadequate protection from the whiplash injuries that can result, according to analysis by Consumer Reports and crash-test data.

The rapid snapping back of a person’s head during a collision can hyperextend the neck and damage nerves and ligaments, often leaving the victim with persistent pain and limited mobility.

Many whiplash injuries could be minimized or prevented altogether with proper head restraints and seatbacks, especially for backseat passengers. CR’s researchers found that automakers are inconsistent in providing effective head restraints in all seating positions. And auto-safety advocates tend to focus on other concerns, largely because “rear-enders” rarely are life-threatening. Compounding the problem: Even if their cars have adequate restraints, most people don’t know how to properly position them or don’t take the time to do it.

Here are some suggestions on what to look for in a vehicle, what to avoid and how to protect yourself from neck injuries in the event of a crash.

Proper combination

Since 1969, the government has mandated that all passenger cars have head restraints on outboard front seats. But tests conducted by the Insurance Institute for Highway Safety (IIHS) to determine rear-crash protection have found that even head restraints that are properly positioned don’t necessarily provide good protection. Of the approximately 175 vehicles for which the institute now has overall ratings, only about one-third are rated “Good” or “Acceptable.” Nearly a third are rated “Marginal,” and more than a third are rated “Poor.”

What’s needed, experts say, is a proper seatback/head-restraint combination that cushions your shoulders and allows your torso, neck and head to move in the same position relative to each other as they would in a natural sitting position. The top park of the seatback, where your shoulders hit it, should be soft and pliable. If you can’t sink into the seat, you rebound off it during the crash, and that can cause the injury. The top of the head restraint, meanwhile, should reach at least as high as the top of your ears and be relatively close – 3 inches or less – to the back of your head.

Rear-seat woes

There are even more problems in the rear of a vehicle, where CR has found that many outboard seats use integral restraints, which often are little more than bumps on the top of the seatback. Many vehicles also lack head restraints for the center-rear position, which isn’t required by the U.S. government. To be effective in preventing whiplash, CR advises that rear head restraints must stay fixed at least 29.5 inches above the seat cushion. Of the 2007 vehicles CR recently tested, only a few more than half had restraints in the rear outboard positions that are tall enough without adjustment. Just half had any restraints in the rear-center position.

Position self for safety

If you’re in the market for a car, check with the IIHS (www.iihs.org) for its front-seat ratings, and with CR’s monthly auto-test reports (www.consumerreports

.org) for CR’s rear-seat head restraint evaluations. Whatever car you drive, always wear your safety belt; it helps you stay in position during a crash. Other steps to take:

l Sit upright. The head restraint can’t help if you are leaning to one side when a crash occurs.

l Brace for impact. If you see a crash coming – or hear the squeal of tires behind you – and have time to react, lean back so that your head is touching the head restraint and look straight ahead. This will minimize any whiplash effect.