Seven years after serious shortcomings in the rear underride guards of most semitrailers were found, the eight largest North American manufacturers are now making rear guards capable of preventing deadly underride crashes in a range of scenarios.

Underride crashes occur when a passenger vehicle slides under a larger vehicle. This usually causes severe intrusion into the passenger vehicle’s occupant space and is often deadly. Rear underride guards are metal bumpers that hang from the backs of trailers to prevent underride in a rear impact.

Trailers on U.S. roads must have underride guards that meet federal safety standards, and guards from the major manufacturers also meet a more stringent Canadian regulation. However, research from the Insurance Institute for Highway Safety (IIHS) showed that even guards that met these requirements could buckle or break off in a crash.

Trailers that qualify for the IIHS’s Toughguard award have rear guards that prevent underride of a midsize car in three test modes — full-width, 50% overlap and 30% overlap.

In each configuration, a typical midsize car travels at 35 mph toward the back of a parked semitrailer. In the full-width test, which all trailers were able to pass in the initial round of testing, the car strikes the center of the guard head-on. In the 50% overlap, which all but one trailer passed initially, half of the car’s front end strikes the guard. In the 30% overlap, the toughest evaluation, 30% of the car’s front strikes the corner of the trailer.

In 2016, 424 of the 2,056 passenger vehicle occupants killed in large truck crashes died when their vehicles struck the rear of a large truck. It’s not known how many of these were underride crashes, according to IIHS. However a 2010 analysis by the group of a small sample of fatal crashes involving the rear of a truck found that 82% involved underride.

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