ARLINGTON, Va. — Modern tractor-trailers for the most part do a good job of keeping passenger vehicles from sliding underneath them, greatly increasing the chances of surviving a crash into the back of a large truck, recent tests by the Insurance Institute for Highway Safety (IIHS) show.
But in crashes involving only a small portion of the truck's rear, most trailers fail to prevent the potentially deadly under-riding, IIHS said.
Under-ride guards are steel bars that hang from the backs of trailers to prevent the front of a passenger vehicle from moving underneath during a crash.
Most trailers are required to have the guards.
Earlier research showed that the minimum strength and dimensions required for under-ride guards are inadequate, prompting the Institute to petition the National Highway Traffic Safety Administration in 2011 for tougher standards.
The Institute also asked the agency to consider applying the standards to other types of large trucks such as dump trucks that aren't required to have any under-ride guards, the Institute said in a news release.
Although NHTSA hasn't responded yet, trailer manufacturers already are installing guards that are much stronger than the agency requires and these guards generally work well to prevent under-ride, except in crashes occurring at the outer edges of trailers, the crash tests show.
One likely reason manufacturers are installing guards that are stronger than required is a tougher standard that trailers in Canada have had to meet since 2007.
More recently, IIHS crash tests have drawn attention to the issue, and at least one manufacturer has started selling a trailer with an improved under-ride guard since the tests began, according to David Zuby, the Institute's chief research officer.
To see how well the latest guards work, IIHS engineers put trailers from the eight largest manufacturers through a series of progressively tougher crash tests. All of the trailers had under-ride guards that met both U.S. and Canadian standards. Both standards require a guard to withstand a certain amount of force at various points. Under the Canadian regulation, a guard must withstand about twice as much force as required by the U.S. rule at the point where it attaches to its vertical support.
In each crash test, a 2010 Chevrolet Malibu struck a parked truck at 35 mph.
In the first scenario, the car was aimed at the center of the trailer. All eight guards successfully prevented under-ride, including one from Hyundai Translead, whose previous model failed a full-width test by IIHS. In the second test, in which only half the width of the car overlapped with the trailer, all but one trailer passed. However, when the overlap was reduced to 30 percent, every trailer except one from the Canadian manufacturer Manac failed. Manac sells dry van trailers in the U.S. under the name Trailmobile. The Institute uses a 30 percent overlap for the most challenging under-ride test because it is the minimum overlap under which a passenger vehicle occupant's head is likely to strike a trailer if an under-ride guard fails.
"Our tests suggest that meeting the stronger Canadian standard is a good first step, but Manac shows it's possible to go much further," Zuby said.
All the improvements in occupant protection that have helped drive down crash deaths in recent decades count for little when the front of a passenger vehicle ends up under a truck, Zuby said.
When this happens, the top of the occupant compartment gets crushed because the structures designed to absorb the energy of a crash are bypassed. The airbags and safety belts can't do their jobs, and people inside can experience life-threatening head and neck injuries.
The crash tests show how this occurs. The 2010 Malibu was an IIHS Top Safety Pick, and in the 40 mph moderate overlap barrier test used to evaluate the Malibu's frontal crashworthiness, measurements recorded by a dummy in the driver seat indicated serious injuries were unlikely. Similarly, in the under-ride tests in which the guards held up, the Malibu's structure and airbags protected the dummy, and injury measures were generally low and not life-threatening. In contrast, when the guards failed, head and neck injury measures were so high that real drivers would have died.
After the Manac trailer 30 percent overlap test
The under-ride guard on the Manac trailer, above, was the only one of eight tested to prevent under-ride in a 30 percent overlap crash. The performance of the Utility trailer, below, was more typical.
After the Utility trailer 30 percent overlap test
In 2011, 260 of the 2,241 passenger vehicle occupants killed in large truck crashes died when the fronts of their vehicles struck the rears of trucks. That's down from 460 out of 3,693 in 2004. The decline is likely due in part to changes in both truck and passenger vehicle traffic resulting from the weak economy.
Gaps in federal crash data make it difficult to pinpoint exactly how many of these crashes involve under-ride. A 2011 IIHS study of 115 crashes in which a passenger vehicle struck the back of a heavy truck or semitrailer found only about one-fifth involved no under-ride or negligible under-ride. Nearly half of the vehicles had severe or catastrophic under-ride damage, and those vehicles accounted for 23 of the 28 fatal crashes in the study.
Crash test results
The Institute previously released the results of an initial round of crash testing on three semitrailers conducted in 2010 and 2011. The weakest guard tested at that time was from Hyundai. It met the U.S. standard for strength but not the Canadian one. When the Malibu hit the center of the Hyundai trailer at 35 mph in a full-width crash, the guard broke, resulting in catastrophic under-ride. In 50 percent overlap tests, the under-ride guard on a Vanguard trailer allowed moderate under-ride at 25 mph and severe under-ride at 35 mph.
In contrast, a Wabash trailer had no under-ride in either the full-width or the 50 percent overlap test. However, when the Wabash was put through a 30 percent overlap test, the under-ride was catastrophic. That's because the Malibu hit the guard outside its vertical attachment bar, causing the unsupported end of the guard to bend forward. The Wabash under-ride guard hasn't been redesigned since then.
Since the initial evaluations, IIHS tested Hyundai and Vanguard trailers again after those companies made changes to their under-ride guards. Trailers from five more companies also were tested. All eight manufacturers now have under-ride guards meeting the Canadian standard, and none of the current designs had any difficulty passing the full-width test.
Most passed the 50 percent overlap test, too. The exception was the Vanguard. The guard's vertical support broke off the trailer when the guard was hit by the car, just as it did in the test of the previous design.
"Vanguard's older and newer under-ride guards were certified to the Canadian standard, so clearly the Canadian regulation, while an improvement over the U.S. rule, isn't stringent enough," Zuby says. "Failing the 50 percent test is a big problem because in our analysis of real-world crashes with the rears of trucks, about half of those with severe under-ride had overlaps of 50 percent or less."
Although the rest of the trailers passed the 50 percent overlap test, most had the same difficulty with the 30 percent overlap that the Wabash trailer experienced in the initial round of testing. The problem stems from the location of the under-ride guards' vertical supports. On most trailers, the supports are attached to the slider rails, which run lengthwise under the trailer and allow the position of the wheels to be changed depending on the load. Using this structure as the under-ride guard's attachment point means the vertical supports are located an average of 28 inches from the trailer's edge.
Manac, the only trailer to pass the 30 percent test, takes a different approach. The supports of its under-ride guard are attached to a reinforced floor and spaced just 18 inches from the edge. The Malibu and the dummy inside it not only fared better, but the Manac trailer also had damage estimates among the lowest of all the trailers. It required only a replacement under-ride guard.
"If trailer manufacturers can make guards that do a better job of protecting passenger vehicle occupants while also promising lower repair costs for their customers, that's a win-win," Zuby says. "While we're counting on NHTSA to come up with a more effective regulation, we hope that in the meantime trailer buyers take note of our findings and insist on stronger guards."
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