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New Study Finds Big-Rig Underride Guards "Inadequate"

3/19/2013
Most modern semi-trailers are required by law to have underride guards, steel bars that hang from the backs of the trailers and which are designed to prevent passenger vehicles from sliding beneath the trailer in a crash.

This is an important safety design because when a passenger vehicle impacts the back of a semi-trailer, the trailer's load floor is approximately the same height as the passenger vehicle's windshield, and the trailer's wheels are not always located under the rearmost portion of the load floor. A passenger vehicle impacting the back of a semi-trailer without underride guards would take the brunt of the force at the windshield, bypassing structures and systems designed to protect occupants, leaving them vulnerable to serious injury or death. A passenger vehicle impacting the back of a semi-trailer with underride guards is expected to deploy safety systems and engineering solutions to reduce the chance of serious injury or death.

A new study conducted by the Insurance Institute for Highway Safety (IIHS) finds that modern semi-trailers do a good job of keeping passenger vehicles from sliding beneath them when the impact is centered on the underride guard, "greatly increasing the chances of surviving a crash into the back of a large truck." However, if a passenger vehicle impacts only a small portion of the underride guard to the left or right of center, the IIHS concludes, "...most trailers fail to prevent potentially deadly underride."

The IIHS tested eight trailers from the largest semi-trailer manufacturers, all equipped with underride guards that meet current regulations in the United States and Canada. It should be noted that Canadian regulations are stricter than U.S. regulations.

The IIHS conducted a total of 24 crash tests, using previous-generation 2010 Chevrolet Malibu midsize family sedans (the Malibu was redesigned for the 2013 model year). In each test, a Malibu was crashed into the back of the trailer at 35 mph. Tests in which the Malibus crashed into the center of the trailer demonstrated that all eight trailer designs successfully prevented vehicle underride. Tests in which the vehicles crashed into 50 percent of the underride guards demonstrated that all except for one of the trailers successfully prevented vehicle underride. The trailer that failed this test is built by Vanguard.

In its third round of testing, the IIHS reduced the degree of overlap from 50 percent to 30 percent "because it is the minimum overlap under which a passenger vehicle occupant's head is likely to strike a trailer if an underride guard fails." When an underride guard fails, the IIHS says, the top of a passenger vehicle's occupant compartment is crushed. In this test, every trailer failed except one sold by Manac under the brand name Trailmobile.

The only trailer manufacturer to pass all three tests, Manac designs its underride guard supports to attach to a reinforced trailer floor and spaces them 18 inches from the edge of the trailer. In the 30-percent offset test with the Manac Trailmobile trailer, the Malibu and the crash-test dummy fared better. Additionally, the Manac Trailmobile trailer had damage estimates among the lowest of all of the trailers.

"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 David Zuby, the Institute's chief research officer. "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."

In 2011, passenger-vehicle crashes with large trucks killed 2,241 people, with 260 of the fatalities the result of passenger vehicles striking the rear of a truck. A 2011 study conducted by the IIHS of 115 crashes involving passenger vehicles and large trucks showed that nearly half of the vehicles involved had suffered severe or catastrophic underride damage, resulting in 23 fatalities.

The IIHS has petitioned the National Highway Traffic Safety Administration (NHTSA) for tougher semi-trailer underride standards, and to require other types of large trucks to have underride guards, but says that the NHTSA hasn't responded. "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," Zuby said.

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