Based on new crash test results, the Insurance Institute for Highway Safety (IIHS) has determined that major semitrailer manufacturers need to improve the underride guards that are installed on the backs of semitrailers to further increase the potential for motorist survival following a collision. Underride guards are the steel bars that hang down from the rear of the trailer (often wrapped in red-and-white reflective material) and are designed to prevent a car from sliding under the trailer in a collision. Most modern semitrailers are required to have them. Since 2007, Canadian regulations require them to withstand up to twice as much force as is required under U.S. regulations.

The latest round of semitrailer crash test results indicate that while trailer manufacturers are doing a good job of protecting motorists who crash directly into the trailer's underride guard -- as well as those whose passenger vehicles impact an underride guard with 50 percent overlap or greater -- a significant threat remains when a vehicle hits an underride guard with 30 percent overlap or less.

To conduct the tests, the IIHS crashed a 2010 Chevrolet Malibu midsize sedan into eight different trailers from major semitrailer manufacturers, each one equipped with underride guards designed to meet both U.S. and Canadian regulations. Each semitrailer was subjected to three separate tests, with a Malibu striking the underride guard from three separate approaches at a speed of 35 mph. The 2010 Malibu was named a top pick by the IIHS when the car was new.

In the first test, the Malibu collided with the center of the semitrailer's underride guard, and all eight of the guards successfully prevented the family sedan from sliding beneath the trailer.

For the second test, the Malibu struck the semitrailer's underride guard with 50 percent overlap, meaning that half of the car's front end impacted the guard. In this test, all but one of the guards successfully prevented the Chevy from sliding beneath the trailer. The trailer that failed this test -- because the underride guard's vertical support broke off when the Malibu struck the trailer -- was built by Vanguard.

"Vanguard's older and newer underride guards were certified to the Canadian standard, so clearly the Canadian regulation, while an improvement over the U.S. rule, isn't stringent enough," said David Zuby, chief research officer for the IIHS. "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 underride had overlaps of 50 percent or less."

The third test measured what occurred when the Malibu struck the semitrailer's underride guard with 30 percent overlap, meaning that slightly less than one-third of the car's front end impacted the guard. In this test, every semitrailer except for one failed to protect the Malibu's driver from serious injury or death. Manac, a Canadian firm that sells trailers in the U.S. under the brand name Trailmobile, built the trailer that passed this test.

"Our tests suggest that meeting the stronger Canadian standard is a good first step, but Manac shows it's possible to go much further," said Zuby.

In 2011, the IIHS petitioned the National Highway Traffic Safety Administration (NHTSA) to develop tougher standards for semitrailer underride guards, and has asked the NHTSA to consider applying the standards to other types of semitrailers that are not currently required to have them, such as dump trucks. As this article is written, the NHTSA has not responded to the petition from the IIHS.

When an underride guard fails to prevent a passenger vehicle from sliding under a semitrailer, the top of the vehicle (the upper portion of the hood, the windshield pillars and the windshield itself) absorbs the brunt of the impact. Because the structures designed to absorb crash energy are bypassed in such instances, the airbags and seat belts are rendered less effective, and the people inside of the vehicle are more likely to suffer life-threatening head and neck injuries. Based on the data collected during this most recent round of semitrailer underride guard crash tests, the IIHS has determined that in the cases where the semitrailer's guard failed, the Malibu's driver would have died.

There is an additional benefit to stronger semitrailer underride guards, as illustrated by the Manac trailer tested by the IIHS. The damage estimates to the Manac trailer following the 30 percent overlap test were among the lowest of all the trailers tested, because all it required was a new underride 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."

author photo

Christian Wardlaw is passionate about the cars, trucks, and SUVs people actually buy, not the models about which they fantasize. An industry veteran and former editor-in-chief of Edmunds.com, this father of 4 loves to inform and entertain everyday car buyers.

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