Accidents involving semi trucks and 18-wheelers are devastating, particularly because of the size and weight of trucks. But a heavy semi doesn’t have to hit a smaller car for the crash to be deadly: often, crashes in which a car smashes into the back of a semi can be fatal to the car’s occupants. This is because the car slides underneath the semi, causing the back of the semi to go through the front windshield of the car, crushing the driver and occupant compartments.

For this reason, most of today’s semis have mandatory “underride” guards, which are steel bars that are supposed to prevent a car from sliding under the semi in the event of a rear-end accident.

Unfortunately, however, recent tests by the Insurance Institute for Highway Safety have found that “in crashes involving only a small portion of the truck’s rear, most trailers fail to prevent potentially deadly underride.”

In the tests by the IIHS, a 2010 Chevrolet Malibu crashed into a parked truck while traveling at 35 miles per hour. When the car crashed into the center of the semi, all of the eight semis passed the test. When the car crashed into the semi with an overlap of 50 percent, seven semis passed. But when the overlap was decreased to 30 percent, just one semi passed. According to the IIHS, “The Institute uses a 30 percent overlap for the most challenging underride test because it is the minimum overlap under which a passenger vehicle occupant’s head is likely to strike a trailer if an underride guard fails.”

In the tests in which the underride guards failed, the dummy’s injury measures indicated that a real driver would have died.

The IIHS hopes that semi manufacturers will take note of their findings and require stronger guards. Though many semi trucks already install guards stronger than required in order to meet tougher Canadian standards, they are obviously not strong enough to prevent injury to other drivers.