NEW YORK (CNNMoney.com) -- Seat and head restraints in more than 60 percent of car models fall short of state-of-the-art protection for neck injuries and whiplash, a new study has found.
Only 22 of 75 cars tests rated good for rear crash protection in collisions, according to a new test of car seats by the Insurance Institute for Highway Safety (IIHS), an industry group.
The institute says that neck injuries are the most common injuries reported in automobile crashes, estimating that they account for 2 million insurance claims each year, costing at least $8.5 billion.
The institute's ratings were based on geometric measurements of seats and head restraints, as well as simulated crashes at 20 miles per hour that measure how well seats would protect people of different sizes in rear-end collisions.
The number of good ratings, at 22, was an improvement from 2004, the first year that IIHS conducted the study, when seats in only eight car models earned the distinction.
12 models in the study earned an acceptable rating, while 17 earned a poor rating.
Volvo turned in the best result of any car maker, with all its models scoring good ratings. But all the Buick and Cadillac models in the study earned a poor rating.
"Even though we have more good performers, it's disappointing that so many designs are still rated marginal or poor," said Institute president Adrian Lund in a statement. "Neck injuries are common in crashes, and it's not difficult or expensive to design more protective seat/head restraints."
The institute awarded higher ratings to designs that contacted the crash test dummy's head quickly, and lessened forces on the neck and torso.
The best designs for neck protection included the Volvos, the Audi A4, S4 and A6; the Ford Five Hundred and Mercury Montego; Nissan Sentra and Versa; Saab 9-3; and Subaru Impreza and Legacy/Outback.
Several manufacturers improved their ratings from poor to good this year. Restraints in the Audi A4 and S4, Honda Civic, Hyundai Sonata, Kia Optima and Nissan Sentra achieved the turnaround.
Some car makers actually earned lower ratings than in previous tests. Restraints in the Chrysler 300, Kia Amanti and Nissan Altima declined to marginal from acceptable ratings in earlier tests.
"We're simulating what happens when a vehicle rear-ends another one in commuter traffic or at a stop light," said Lund. "People think of head restraints as head rests, but they're not. They're important safety features."
When a car is rear-ended, the vehicle's seat propels the occupant's body forward, and if the passenger's head is not supported, it will lag behind. The goal of a head restraint is to move the occupant's head at the same speed as the torso.
Head restraints should extend as high as the center of gravity for the head of the expected occupant, and be positioned close to the head.
The institute measures the geometry of head restraints first to make sure they could protect taller people. If cars don't meet the bare minimum for geometry, they aren't included in the crash tests.
Models that didn't pass the initial geometry test include the Cadillac DTS, Pontiac Grand Prix, Suzuki Forenza and Reno, plus some seats in the BMW 5 series, Buick Lacrosse and Mitsubishi Galant.
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