ARLINGTON, VA — Modern semi-trailers do a good job for the most part in keeping cars from sliding underneath them in a collision, according to recent tests of van trailers by the Insurance Institute for Highway Safety (IIHS, the organization that, among other things, tests cars for ‘crashability’). However, in crashes involving only a small portion of the truck’s rear, most trailers fail to prevent potentially deadly car-passenger injury.

The sole exception in these tests was a trailer built to the Canadian specification and a bit beyond, says the IIHS. Ours is a tougher standard than the American one that trailers in Canada have had to meet since 2007. In one test performed by the IIHS in which only 30 percent of a car hits a van trailer’s rear, a guard that exceeded the Canadian spec — and was manufactured here in that case — was alone in passing the test.

The basic Canadian standard creates guards that absorb much more energy, and in theory U.S.-based trailers entering Canada must have them. The problem is that they all look the same, unless you examine the certification label that’s affixed to them. It’s unknown how many American trailers enter Canada without compliant guards.

Most trailers in North America are required to have under-ride guards at the rear to prevent the front of a passenger vehicle from plowing underneath during a collision. Earlier IIHS research showed that the minimum strength and dimensions required for under-ride guards are inadequate, prompting a petition to the National Highway Traffic Safety Administration (NHTSA) in 2011 for tougher standards.

NHTSA hasn’t responded yet, but some trailer manufacturers are already installing guards that are much stronger than the agency requires. These guards generally work well to prevent underride, says the IIHS, except in crashes occurring at the outer edges of trailers, the tests show.

IIHS tests done two years ago, along with the more recent ones, have drawn attention to the issue. As a result, some U.S. manufacturers have voluntarily started selling a trailer with an improved underride guard built to the Canadian spec.

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 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, the IIHS explains, 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. 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 a Canadian manufacturer failed. And its under-ride guard is built to exceed even the tougher Canadian standard.

The Institute says the 30 percent overlap is the most challenging under-ride test because it’s the minimum overlap under which a car occupant’s head is likely to strike a trailer if the guard fails.

“Our tests suggest that meeting the stronger Canadian standard is a good first step, but… it’s possible to go much further,” says David Zuby, the Institute’s chief research officer.

A key issue appears to be the location of the guard’s vertical supports. On most trailers, the supports are attached to the slider rails, which run lengthwise under the trailer. Using this structure as the under-ride guard’s attachment point means the vertical supports are located an average of 28 in. from the trailer’s edge, says the IIHS.

The only trailer to pass the 30 percent test, Canadian-made, had its guard supports further outboard, attached to a reinforced floor and spaced just 18 in. from the edge. Both the Malibu and its crash-test dummy fared better but so did the trailer, which had damage estimates among the lowest of all the trailers tested. 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.”

You can access the full report here. — RL