When an automobile collides with the rear of a semi-truck trailer it can cause devastating injuries and fatalities. The increased risk is a result of the height of the trailer compared to the car. The hood of most passenger cars are low enough to slide below the deck of the trailer so that the force of the impact goes directly into the windshield and upper part of the passenger compartment of the car. These types of crashes are known as 'underride' accidents.
Because of this risk semi trailers have a device called a rear impact guard which extends downward from the very back of the trailer. The guard is supposed to act like a bumper to stop the grill area and hood of the car from sliding underneath the back of the trailer. While this can still result in a serious accident, it allows the front of the car to absorb the energy of the crash without compromising the passenger compartment. Unfortunately a new test from the Insurance Institute for Highway Safety (IIHS) indicates that the guards may not be strong enough to prevent the car from sliding under the semi even at moderate speeds.
For the test, the IIHS crashed a 2010 Chevrolet Malibu into the back of a stationary semi trailer that was fitted with the rear impact guard. The car pushed right through the guard and the trailer came crashing through the windshield. Had there been actual occupants in the car rather than crash test dummies, this guillotine action would have caused catastrophic injuries. In comparison they also tested a guard which was 75 percent stronger, meeting the more stringent Canadian requirements, that guard effectively stopped the car before it slid under the semi.
According to report from ABC News, more than 350 people are killed each year in these types of underride crashes. The National Highway Traffic Safety Administration says that is has been aware of this issue since 2009 and continues to investigate a way to address this issue.
Source ABC News "Truck Underride Accidents: Drivers Endangered When Cars Slide Under Trailers" Lisa Stark, March 1, 2011