In collisions where passenger cars pile head-on into the rear of semitrailers, the protective steel underride guards required on most tractor-trailers prevent cars from sliding underneath them better than they used to. But in offset collisions, where the impact overlaps half the car's width or less, a major risk still exists because the guards bend forward.
The Insurance Institute for Highway Safety's latest test elaborates on a test in 2011 when one semitrailer failed even the full-frontal test. IIHS petitioned the National Highway Traffic Safety Administration to create tougher standards, noting NHTSA's estimate that impacts to the rear of large trucks cause 423 deaths and more than 5,000 injuries to passenger-car drivers each year. However, IIHS reports that the federal agency has yet to update its standards.
The 2011 test evaluated three semitrailers; the latest test evaluates eight. As before, IIHS crashed a fleet of 2010 Chevrolet Malibus into semitrailers at full-overlap — essentially hitting the center of the trailer's rear end — plus offset overlaps of 50% and 30% of the Malibu's width. All eight trailers passed the full-overlap test; seven passed the 50% offset. Just one semitrailer, from Canadian company Manac, passed the 30% overlap thanks to underride supports that sit farther outboard than in the other trailers. On the other trucks, the guard supports sat too far away from the point of impact. They bent forward to allow the Malibu to slide underneath the semitrailer.
It doesn't take a Rhodes scholar to see why the results are catastrophic. Automakers design cars to absorb most impact forces through steel framework under the hood — not the windshield and A-pillars, which take the brunt of the impact when a car slides under a semitrailer. IIHS notes that since 2007, Canada has had considerably tougher underride standards than the U.S. All eight trailers met the Canadian standard — a contrast to 2011 when only two of the three trailers tested met it — but neither U.S. nor Canadian standards require wide enough underride supports to prevent a slide-under. IIHS says 260 passenger-car occupants died in 2011 when their cars rear-ended a semitrailer. That's roughly 12% of all passenger-vehicle occupant deaths when a passenger vehicle and large truck crashed.
Offset collisions are common if you try — unsuccessfully — to steer clear of the impact. IIHS has championed such evaluations in its other tests, with new small-overlap frontal tests that simulate hitting a barrier that overlaps just 25% of the car.