NEW YORK (CNNMoney.com) -- Two-thirds of pickups, vans and sport-utility vehicles don't provide acceptable protection against whiplash in rear-end collisions, according to tests by the Insurance Institute for Highway Safety (IIHS).
Some 54 models were rated "marginal" or "poor" by the Institute in tests that involved measurements of seats and head restraints and, in some cases, simulated rear-end collisions.
Vehicle seats and head restraints are first measured to determine if the headrest, because of its shape and angle, is likely to provide adequate protection.
IIHS: Top Safety Picks
Seats that are deemed likely to provide good protection are then tested in a simulated crash. The seat is attached to a moving platform that is struck from behind at 20 miles per hour. The Institute then measures impact forces on a crash test dummy riding in the seat.
Twenty-one current models - or 24 percent of those tested - earned the best possible rating of "Good." Twelve models earned an "Acceptable" rating, the second-best rating.
"In stop-and-go commuter traffic, you're more likely to get in a rear-end collision than any other crash type," says IIHS's David Zuby. "It's not a major feat of engineering to design seats and head restraints that afford good protection in these common crashes."
The agency stresses that to reduce neck injuries, the vehicle's head restraint should be high enough to be at the back of the head.
Rear-end collisions are frequent, and neck injuries account for 2 million insurance claims each year, costing at least $8.5 billion.
For SUVs, the best performers were models by Subaru, Volvo, Acura, Ford, Honda and Hyundai.
Some manufacturers objected to the fact that the seats are not tested while mounted in the vehicle.
"We are confident our whiplash injury lessening system and vehicle design will provide a reasonable level of safety for our customers in the real world," Toyota said in a statement.
While Toyota's Tundra pickup earned the top "Good" rating for whiplash protection in the IIHS tests but several other Toyota vehicles rated "Poor."
General Motors also issued a statement saying: "Head restraints are part of the integrated approach to occupant protection in all GM vehicles." GM's statement also noted that the company designs its head restraints for drivers of various sizes rather than a single set of measurements.
The best rating for a GM SUV, truck of van was an "Acceptable" rating for the Chevrolet Silverado pickup. Other GM vehicles in the tests rated "Marginal" or "Poor."
Overall, SUVs have the most vehicles with "Good" protection - 29 percent - while vans followed with 27 percent and only 6 percent of pickups earned a "Good" rating.
"The reason may be that automakers have updated or introduced many new SUVs since 2006, but minivans and pickups are being updated more slowly," said Zuby. 
