Canadian Underwriter

Semitrailer underride guards put passenger vehicle occupants at risk in certain crashes: U.S. institute

March 15, 2013   by Canadian Underwriter

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Most trailers on modern semitrailers fail to prevent incidents where a passenger vehicle slides underneath the rig in collisions involving only a small portion of the truck’s rear, indicate new crash test results from the Insurance Institute for Highway Safety (IIHS).


The tests show that “modern semitrailers, 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,” Arlington, Virginia-based IIHS reported yesterday.

An underride guard is a steel bar that hangs from the back of a semitrailer to prevent the front of a passenger vehicle from moving underneath during a collision. Crash tests show these guards generally work well to prevent potentially deadly underride, except in crashes occurring at the outer edges of trailers, notes a press release from the institute.

Trailer manufacturers are already installing guards that are much stronger than required by the National Highway Traffic Safety Administration (NHTSA) in the United States, the IIHS reports. “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.”

In the latest round of testing, IIHS engineers put trailers from the eight largest manufacturers through a series of progressively tougher crash tests. All trailers had underride guards that met U.S. and Canadian standards, both of which require a guard to withstand a certain amount of force at various points. The Canadian regulation requires a guard to withstand about twice as much force as the U.S. rule at the point where it attaches to its vertical support.

The tests involved a 2010 Chevrolet Malibu – an IIHS Top Safety Pick – striking a parked truck at 35 mph (56 kph). When the vehicle hit the centre of the trailer, all eight guards prevented underride; when half the width of the car overlapped with the trailer, all but one trailer passed; and when overlap was reduced to 30%, every trailer except one from the Canadian manufacturer Manac failed.

With regard to the 30% results, IIHS notes the problem stems from the location of the underride 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 underride guard’s attachment point means the vertical supports are located an average of 28 inches (70 cm) from the trailer’s edge.”

IIHS points out that Manac employs a different approach by having underride guard supports attached to a reinforced floor and spaced just 18 inches (45 cm) 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 underride guard,” the institute reports.

“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,” suggests David Zuby, chief research officer for IIHS.

“In the underride 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,” the institute reports. “In contrast, when the guards failed, head and neck injury measures were so high that real drivers would have died.”

The IIHS notes that improvements in occupant protection “in recent decades count for little when the front of a passenger vehicle ends up under a truck. 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.”

In 2011, 260 of the 2,241 passenger vehicle occupants killed in large truck crashes died when the fronts of their vehicles hit the rears of trucks. Although 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,” notes the IIHS. Gaps in federal crash data make it difficult to pinpoint exactly how many of these crashes involve underride, the institute adds.

The IIHS petitioned the NHTSA in 2011 for tougher standards and requested that the agency consider applying the standards to other types of large trucks for which underride guards are not mandated. A response has not yet been received, says the institute statement.

“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 suggests.

One guard failed at the 50% test, despite its underride guards being certified to the Canadian standard. “So clearly the Canadian regulation, while an improvement over the U.S. rule, isn’t stringent enough,” Zuby emphasizes.

“Failing the 50% test is a big problem because in our analysis of real-world crashes with the rears of trucks, about half of those with severe underride had overlaps of 50% or less.”

Says Zuby, “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.”