Why Do Semis Have a Metal Bar Hanging from the Back of the Trailer?

Photo of a Tractor Truck Trailer Underride Guard

If you’ve ever been stuck behind a semi-tractor trailer on a highway, you have probably noticed a long steel bar hanging off the back of the trailer. What is exactly the purpose of the bar? Well, it’s actually one of the most important safety features on the trailer.

Those steel bars hanging from the truck’s platform bed are known as a “rear underride guards” or “rear impact guards.” They are designed to prevent vehicles from sliding underneath a trailer during a rear-end collision.

Unlike your average car, truck trailers and straight trucks do not have rear bumpers. In a typical car vs. car rear-end collision, both vehicles’ bumpers help to absorb the impact of the crash. That absorption happens because car bumpers minimize height differences of any two vehicles.

How underride guards save lives

Obviously, the back ends of trailers are much taller than the bumper of your sedan. Why? Trucks are routinely backed up to uniform-height loading docks. To accommodate, most truck trailers sit 48 inches off the ground. In contrast, cars and other passenger vehicles have a height between 16 and 20 inches off the ground.

If a car crashes into the rear of a semi-tractor’s trailer, the entire front of the car could slide right underneath the trailer bed. The best bumper force-absorption technology in the world would still be completely useless in this situation.

The height mismatch between the two vehicles is so great that neither bumper would ever hit each other. Unfortunately for many car passengers, the height of the average trailer bed lines up with the head of a seated adult in an average sedan. This problem leads to many serious injuries and fatalities even at low speeds.

Jayne Mansfield and the Origin of Truck Trailer Underride Guards

In 1967, a Hollywood actress named Jayne Mansfield died in a rear-end collision with a tractor-trailer. Her vehicle slid underneath the bed of the truck. As a result, her entire vehicle roof was sheared off, almost killing her and two other adult passengers instantly. Following her death, the National Highway Traffic Safety Administration (NHTSA) made it mandatory for all truck trailers to have rear impact guards. Because of this, rear underride guards are sometimes referred to as “Mansfield bars.”

In 1996, the NHTSA upgraded the required rear underride guard regulations to improve the standardized production quality. There are many YouTube videos like this one that highlight how big of a difference rear underride guards can make.

Side underride guards: The new safety frontier

In recent years, several truck safety advocacy groups have encouraged lawmakers and administrators to require side underride guards to protect passenger vehicles from similar problems in side-impact crashes. In March 2019, senators from both sides of the aisle reintroduced legislation that would require trailers to be equipped with side underride guards. As trucking safety advocates ourselves, we hope that the legislation makes roads safer for all of us.

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