The Insurance Institute for Highway Safety unveiled new crash-test results today that highlight the weakness of steel guards installed on the back ends of large truck trailers.
If the guards fail after being struck by a car, "they can allow catastrophic underride where cars slide underneath" the large truck, the institute warned in releasing the results.
The institute said it tested guard-equipped trailers from manufacturers Great Dane, Manac, Hyundai, Stoughton, Strick, Utility, Vanguard and Wabash.
More than 20 tests were performed using trailers from the eight manufacturers, the institute said, but only Manac's guard prevented underride in each of three tests.
In 2011, more than 250 people died in crashes in which passenger vehicles hit the back end of trailers on large trucks, the institute said. It says it has studied underride crashes for more than 30 years.
Decapitation is a serious threat in underride collisions, the institute said.
Cars are designed to handle severe frontal crashes, but if truck guards bend or break, the windshield of the car can become the main impact point, according to the institute. This can result in the top of the occupant compartment being destroyed.
The American Trucking Associations, responding to the test results, says the focus should be on preventing underside collisions.
The underride guard, the group says, should never have to be used.
"More driver education on sharing the road with large commercial vehicles is a must, and promoting greater use of collision avoidance technology in both cars and trucks will also produce results," the associations said in a statement. "Many studies show that as many as three in four fatalities involving cars and trucks are unintentionally initiated or caused by the driver of the car."
The institute petitioned the federal government in 2011 to require stronger underride guards that remain in place during a collision and mandate guards for more large trucks and trailers.
NHTSA has not formally responded, but the agency said it has discussed underride with several American and Canadian trucking groups, including American Trucking Associations and Transport Canada.
On Wednesday, NHTSA published results of a field analysis it did with the University of Michigan Transportation Research Institute "to better quantify and characterize crash factors" such as "impact speed, trailer type/age, crash impact and the amount of underride."
The agency called the study a census of every real-world fatal crash from 2008 and 2009.
NHTSA said in a statement that it will use findings from its field analysis, IIHS testing, international standards and other sources to "inform potential changes to existing federal safety standards."
The agency said it is researching other ways to improve truck safety, including crash avoidance technologies.
"The driving public should know that we are actively working to address the issues raised in IIHS's report and that their safety will always be our top priority," NHTSA Administrator David Strickland said in a statement.
Trailer manufacturers have begun installing guards that are stronger than the agency requires.
The new guards generally work well to prevent underride, except in crashes occurring at the outer edges of trailers, the institute's crash tests show.
The institute used the 2010 Chevrolet Malibu, an IIHS top safety pick, to run through a series of progressively tougher tests in which it struck the rear of parked trucks at 35 mph.
In the first test, the car was "aimed at the center of the trailer," the institute said. All eight guards prevented underride.
During the second trial, with a 50 percent overlap with the trailer, Vanguard's model was the only model that failed.
On the 30 percent overlap collision, Manac, a Canadian manufacturer, was the only model to pass.
The institute said: "The new tests show that while guards are doing a better job than in the past as manufacturers change guards to meet a tougher Canadian standard, most still allow horrific underride when cars hit the ends of the guards even at just 35 mph."