If you’ve just eaten a meal, we’d suggest holding off on watching the video below for a little bit. Why? The Insurance Institute for Highway Safety recently conducted a test of many semi trailers’ safety in a rear-end crash, and the results are a little bit disturbing.
The part in question is the “underride guard,” the metal bar beneath a tractor trailer’s taillights. The purpose of the guard is pretty simple: it’s there to prevent a passenger car from subducting, or being squashed under, the trailer in the event of a rear-end collision. This is critical because, as you can see from the photos, a car that rides under the trailer in the event of a crash ends up with its A-pillars taking the brunt of the force, as opposed to the stronger front end crash zone. If the bottom of the trailer box hits a car’s A-pillars, the front-row passengers will likely die.
The IIHS purchased and crashed a number of 2010 Chevrolet Malibus into the back of a handful of trailers for this test; this being the IIHS, the tests included full overlap (hitting the trailer at 35 mph head-on), half overlap, and 30-percent overlap (similar to the institute’s new small-overlap front crash test).
What the IIHS found is that all of the trailers prevented underride when hit straight-on, but when they introduced some overlap, the crashes went from minor to severe or even deadly. In many cases, hitting the underride guard with some overlap sent too much crash force through one of the bar’s supports, bending or breaking it. In those cases, the trailer ended up in the passenger compartment, likely killing the driver. Only one trailer—the Manac, or Trailmobile, trailer—prevented underride in all tests. The IIHS says this is because Manac’s underride guard supports are spaced farther apart, better distributing crash forces.
In any case, the video was enough to remind us that crashes involving tractor trailers are rarely simple or minor. Perhaps it’s a good reminder that you should always stay out of a trailer’s blind zones, drive distracted, and consider a car with either a forward collision warning or adaptive cruise control when buying a new ride.