“Hitting the back of a large truck is a game changer,” IIHS President Adrian Lund wrote in the report. “You might be riding in a vehicle that earns top marks in frontal crash tests, but if the truck’s underride guard fails—or isn’t there at all—your chances of walking away from even a relatively low-speed crash aren’t good.”

The organization performed six crash tests involving three rear guards that complied with U.S. safety regulations and were attached to parked semi-trailers. In three of the tests, the car slid under the truck enough that the dummy’s head was hit, indicating that decapitation would likely occur in a rear-world crash.

The strongest guard prevented underride when the car struck the truck’s rear head-on and at a slight angle. In every other test in which the car struck the truck at an angle, all of the guards allowed underride.

During the life of a truck, the guards beccome damaged over time. Trucking companies refuse to replace the guards because stronger systems would create a slight increase in weight, which would raise the companies’ fuel costs.

In its rulemaking and research priority plan released last month, NHTSA acknowledged that truck underride is the third largest cause of fatalities in frontal collisions and said it “will assess research data and decide on the next steps” by 2012.