An underride guard is a strong bar made of steel tubing, fitted underneath the rear portion of a semi-trailer to prevent a car from passing underneath the trailer in the event of a rear-end collision. Since a semi-trailer is much higher off the ground than a standard car, without the underride guard, the structural members of the car would pass underneath the structural members of the semi-trailer in a collision. In fact, the structural members of the trailer are roughly at head height with the driver and passengers, with nothing to protect them but the windshield.
The underride guard is also known as a DOT bumper, or a Mansfield bar. The Mansfield bar name comes from a highly publicized accident involving none other than 50's icon Jayne Mansfield, in which her small car rear-ended a large truck, killing three of the six passengers including Jayne Mansfield. This took place on June 29th, 1967 – before The National Highway Transportation Safety Administration required underride guards on semi-trailers – and the car passed under the truck just high enough that Jayne Mansfield's head impacted the rear of the trailer. Popular rumor for quite some time was that she was decapitated, but in reality only a portion of her skull was severed.
Although the primary purpose of the underride guard is to provide something for a car to hit relatively safely in the event of a collision, they are also used to secure a trailer to a loading dock to prevent trailer creep. Trailer creep is the act of a trailer gradually moving away from the edge of the loading dock due to the forces placed on it by forklifts or other loading equipment continuously driving on to and back off of the trailer, which can result in a dangerous gap between the trailer and the loading dock. Although wheel chocks should always be the primary method for preventing trailer creep, a device known as a dock lock can assist them by holding the underride guard with a large steel hook. The biggest advantage a dock lock can provide is realized when loading a trailer with air-ride suspension. Although air-ride suspension provides a smoother ride on the highways, it also causes the trailer to bounce when a heavy object is loaded into it. Repeated bounces can cause the wheel chocks to move, but a dock lock hook would prevent this. Dock lock hooks usually also have an indicator of some sort to let the workers on the dock, who might not be able to see if the wheels are chocked or not, know that the dock lock is engaged.
The National Highway Transportation Safety Administration has set a maximum ground clearance of 22 inches for underride guards. There is no minimum clearance; however if the underride guard is too low it may catch on bumps, debris, and obstructions in the road surface, causing difficulty and discomfort for the driver. Under normal circumstances, 22 inches should be sufficient to impact the structural areas of a car, such as the bumper and crumple zones, but there are situations in which this can fail, creating what is known as the wedge effect.
The wedge effect is created when the underride guard crumples upward into the semi-trailer, or when the car manages to squeeze underneath the underride guard. The underride guard can crumple if it is not adequately braced, or in a very high speed collision. A car can squeeze underneath the guard if the driver realizes a collision is about to take place and applies the brakes forcefully, causing the front end suspension to compress and lowering the front of the car. In either of these circumstances, the car becomes a wedge underneath the rear of the trailer, and can actually lift the trailer into the air and onto the front of the car with a force easily exceeding 10 tons. This can actually be more dangerous than not having an underride guard at all for both the car and the truck.
One proposed solution for this problem is the pliers underride guard. The pliers guard would be secured by a diagonal brace which is free to pivot, and held up by steel cables. This would allow the guard to hang lower to the ground, since it would simply pivot up when encountering an obstruction. When a car impacts the pliers guard, the force of the car on the cables would cause the pivot to swing up into the undercarriage of the car, pinning the front of the car between the underride guard and the bottom of the trailer without the possibility of the car passing under the guard or crumpling the guard up against the trailer.
SharQ tells me of a scene in 2 Fast 2 Furious in which a car becomes trapped underneath a semi-trailer by entering from the side. Underride guards are only installed in the rear of a semi-trailer and provide no protection against accidents from the side. Fortunately, this type of car-truck collision is much more rare.
ke6isf reminds me that one of the major causes of side collisions between cars and trucks is the fact that that trucks are forced into making wide right turns due to their length. A car trying to pass on the right while a truck is making a wide turn could get caught under the trailer.