Underride guards on the backs of large trucks frequently fail to prevent a passenger vehicle from sliding under a truck during a collision, according to a report issued last month by the Insurance Institute for Highway Safety (IIHS).

“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 real-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.

“Damage to the cars in some of these tests was so devastating that it’s hard to watch the footage without wincing,” wrote Lund. “If these had been real-world crashes, there would be no survivors.”

Morgan Adams, a Chattanooga, Tennessee, attorney who has handled many underride cases, said that although the report highlights the dangers of faulty underride guards, the likelihood of injury is even greater than the report suggests.

“The underride guards used in the study are brand new, but in the real world, trucks back up to the loading docks, and the underride devices hit the docks time and time again,” he said. “They are bent, twisted, rusted, scraped, and have already received a huge amount of wear and tear.”

He said trucking companies refuse to replace the guards because stronger systems would create a slight increase in weight, which would raise the companies’ fuel costs.

The IIHS criticized the National Highway Traffic Safety Administration (NHTSA) for a lack of meaningful regulations. The last safety standard governing underride guards was issued more than a decade ago, and it exempted many of the most commonly used heavy trucks, including single-unit vehicles like dump trucks. And guard manufacturers are allowed to test each part—the trailer, guard, bolts, and welding—separately, so there’s no way to know if the guard would be strong enough as a unit, argued the IIHS.

The organization and safety advocates are urging NHTSA to require stronger guards.

“The standard is a farce,” said forensic engineer Roy Crawford, of Whitesburg, Kentucky, who argued that the regulations need to address more than weak underride guards.

Many times, “the trucks are overloaded and going 40 mph below the speed limit. They don’t have enough lights or reflectors, so drivers can’t see them and crash into them,” he said. “There’s an old myth that if you run into something, it’s your fault. But people are just not seeing the trucks.”

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.