Trailer underride guards, while improved, did not pass the Insurance Institute for Highway Safety’s (IIHS) critical 30% test. According to the IIHS, trailer underride guards on modern trailers do a pretty good job at keeping vehicles from sliding underneath them, but primarily when the crash occurs directly behind the trailer. IIHS tests show that when a vehicle strikes a portion of the trailer (overlap), most trailers fail to prevent potentially deadly underrides.
In a IIHS study of 115 crashes in which a passenger vehicle struck the back of a heavy truck or semitrailer, results showed that 80% were underrides. Of those crashes involving underride, 82% were fatalities; about half of those with severe underride had overlaps of 50% or less.
IIHS engineers most recently crash-tested trailers from eight of the largest manufacturers. In each test, a 2010 Chevrolet Malibu struck a parked tractor-trailer at 35 mph. When the car was aimed at the center of the trailer, all successfully prevented the underride. When the mid-point of the car struck the trailer edge, only one guard failed to prevent the underride. However, when the portion of the vehicle striking the trailer was reduced to 30%, all but one failed. The 30% overlap is used by the IIHS for testing because it is the minimum overlap under which a passenger vehicle occupant’s head is likely to strike a trailer in an underride guard failure. It is important to note that in successful tests where the guards held up, the Malibu’s structure and airbags protected the dummy.
Earlier test results from the IIHS showed that the size and strength of the guards were inadequate leading the IIHS to petition the National Highway Traffic Safety Administration (NHTSA) in 2011 for tougher standards. In that set of tests, IIHS engineers crash-tested trailers from three manufacturers (Hyundai, Vangard and Wabash). The Hyundai trailer failed all tests. Vangard failed the 50% and 30% tests while the Wabash trailer failed the 30% test.
Since 2007, under Canadian regulation, a guard must withstand about twice as much force at the point where it attaches to its vertical support compared to the U.S. rule. It’s encouraging to note that while NHTSA has not issued any additional requirements on trailer underride guards, trailer manufacturers have responded to the IIHS results by installing guards that are much stronger than required. All eight manufacturers now have underride guards meeting the Canadian standard, and none of the current designs had any difficulty passing the full-width test.
According to the IIHS, the location of the guards’ vertical supports appears to be a problem. As the supports are attached to the slider rails which allows the position of the wheels to change depending on the load, the vertical supports are located an average of 28 inches from the trailer’s edge. Manac, a Canadian manufacturer and the only one to pass the 30% test, attaches its guards to a reinforced floor and spaced just 18 inches from the edge.