semi truck


The next time you’re sitting behind a semi on the highway, you might notice a long T-shaped bar welded to the back of the trailer. If you’ve wondered why trucks have that metal bar in the back, you’re about to learn why. This device has a purpose. It’s called a “Mansfield bar” or an “underride guard” and serves a very important function on the truck.

This bar is meant to prevent cars from sliding underneath the trailer in the event of a rear-end collision. If a car rear-ends your car, the bumper of your car and crumple zones in the front of the other car absorb the impact and keep the cars from riding over or under each other.

Truck trailers and flatbed trailers don’t have bumpers. They’re also much higher off the ground than most cars. You may have noticed that a modern sedan would fit nicely under the chassis of a semi trailer. If one stopped suddenly in front of you, the lack of a rear bumper means that nothing would absorb the impact, and your car would continue forward under the trailer.

The Gruesome Death of Jayne Mansfield

In 1967, Jayne Mansfield, a blonde bombshell actress in the mode of Marilyn Monroe, died in a hideous rear-end accident with a semi. Her vehicle slid beneath the rear of the truck, shearing off the roof, and killing her and her two adult passengers instantly. Two children sleeping in the back seat survived.

The following year, the National Highway Traffic Safety Administration (NHTSA) mandated all truck trailers must have rear impact guards, and the Federal Motor Carriers Safety Association (FMCSA) has incorporated rear impact guards into their guidelines. Because of the fame of the victim, the guards became known as “Mansfield bars.”

Getting the Bars On the Cars

Trailer beds are 48 inches off the ground, which is industry standard for loading docks. The average sedan hood height is about 20 inches. Unfortunately, the roof height is also about 20 inches, meaning that, as Ms. Mansfield and her passengers learned, the trailer bed is a perfect height to hit the driver of a sedan in the face.

Larger passenger vehicles, like SUVs or Hummers, do not have the specific hazard of underriding that smaller cars do, but are not immune from the slicing damage that will be suffered by striking a steel deck without an impact bar.

As early as 1953, safety-conscious truck manufacturers had begun installing various types of safety bars on the rear of some of their trailers. These were not regulated in any way but anticipated the regulations that were to be issued by the federal government in years to come.

When the rear impact guards regulation went into effect, it was poorly implemented and poorly enforced. Rear-end collisions continue to be a problem. According to a recent report by the Government Accountability Office (GAO), there were on average 219 fatalities in rear-end collisions annually between 2008 and 2017. Whether this is entirely due to improper use of impact guards, or other factors, is unclear.

The State of the Road

There is a new push to try to get trucking companies to add side impact guards to their trailers. In 2019, bipartisan legislation was presented in Congress recommending such requirements. The GAO report examined these suggestions, but discussions with truck manufacturers indicate that there needs to be more study and review before a recommendation mandating side impact guards could be implemented.

One of the problems facing regulators, inspectors, and even drivers is the lack of uniformity in describing an “underride” accident. Not every state defines them in the same way, and not every state reports them the same way—or at all. Guidance added by the FMCSA in 2019 was the first step towards standardizing rear-impact guards since the law was first passed in 1967. Hopefully, this will be the beginning of a trend.

What You Can Do

All safety organizations agree that the best way to avoid serious injury or death in a rear-end collision with a big rig is to avoid being in one. The following distance for a big rig should be approximately 300 feet, or approximately the length of a football field.

If you are trying to “draft” the truck to improve your gas mileage, we have bad news for you. The optimal distance for drafting (as shown in wind-tunnel tests) at 55 miles per hour is about 100 feet, or less than one-third the safe following distance for a big rig. In other words, if you’re trying to be clever and use the truck to help save gas, the only thing you’re doing is risking your life. If the truck brakes suddenly, you simply cannot stop in time to avoid hitting the truck.

To avoid side-impact collisions, minimize your time in a big rig’s blind spots. Don’t pass on the right and pass on the left as quickly as possible. Remember that the driver cannot see you while you are sitting to the left and right rear and may not realize the lane is not empty. Worse, if the semi is riding in the center lane, you could find yourself trapped in the outside lane with no way of escape if the semi swerves in your direction.

If you see a truck driving erratically, give yourself as much distance as possible. Be a good citizen and call 911 or the trucker’s company to report them. You may believe it isn’t your problem, or you don’t want to cause the driver trouble, but it is everyone’s problem if the truck is in an accident twenty miles down the road.

Finally, if you are involved in any kind of accident with a big rig, seek medical and legal advice right away. It can take several hours or days for some types of injuries to develop. Head injuries and neck and back trauma may not appear for some time, so even if you feel okay, don’t wait to see a doctor.

Filing an insurance claim when a commercial truck is involved is not as simple as sending in a single claim. Your injuries may be more serious, the property damage is certain to be total, and there could be multiple parties involved. Consider seeking legal assistance before filing your claims whenever you have been hit by a semi.

Morris & Dewett provides this information to the public for general education and interest. The firm does not represent clients in every topic discussed in legal & injury news. The information is curated and produced based on trends in law, governance, and society to present relevant issues to the general public. Every effort is made to provide accurate information. Do not make any decision solely based on the information provided, please seek relevant counsel for each topic area. Consult an attorney before making any legal decision, consult a doctor before making any medical decision, and consult a financial advisor before making any fiscal decision. If you have any legal needs that we can assist you with, please do not hesitate to contact us.

Morris & Dewett Will Answer Your Questions and Help You Recover