The bar under the back of a trailer has a nickname on the road, taken from a famous death in 1967, and a name in the regulations. The name is the rear impact guard. What it is rated to do is the whole case.
Two federal standards govern it. One describes the guard itself, its strength and the energy it must absorb. The other describes where it sits on the trailer: the bottom edge no more than 560 millimeters off the ground, about twenty-two inches, the ends within about four inches of the trailer’s sides, the face no more than about twelve inches forward of the rear. That geometry is the point. A car’s bumper and the crush structure behind it live down at that height. If the guard is there, the car meets the guard and the front of the car does its job. If the guard is missing, bent up, rusted through, or set too high, the car meets the trailer’s floor at windshield height instead, and the front of the car never touches anything.
The standards apply to trailers of ten thousand pounds and up, built since January 1998, and the Federal Motor Carrier Safety Regulations carry them onto the highway: a trailer must have the guard it was built with, meeting those standards, every day it operates. Older trailers get a looser rule, thirty inches off the ground and within eighteen inches of the sides. Either way it is the carrier’s duty to keep the guard on the truck and in shape, and the annual inspection is supposed to catch the one that is not.
For a long time the guard was built to stop a car at thirty miles an hour. In July 2022 the government raised that. The new rule requires a guard strong enough to protect the occupants of a compact or subcompact car striking the rear of the trailer at thirty-five miles an hour, hitting it squarely and hitting it with half the car’s width. It borrowed the Canadian test to do it: the guard must take a distributed load of 350,000 newtons without bending more than 125 millimeters, while absorbing at least 20,000 joules. Trailers built two years after the rule had to comply.
Read what the rule did not do. Safety groups asked the agency to add a third test, a car hitting the trailer with only thirty percent of its width, the corner hit, which is where the guard is weakest and where a driver who swerves late tends to land. In June 2024 the agency denied that petition and kept the two tests it had. So thirty-five miles an hour, square or half-overlap, is the ceiling of what the standard promises. It is not the ceiling of what happens on Interstate 10 at night.
Now the side of the trailer, where there is no guard rule at all. The agency said so in its own words in April 2023: there are currently no federal requirements for side underride guards on trailers. In the same notice it counted the cost. Eighty-nine deaths a year and 409 serious injuries among people in cars and light trucks that went under the side of a tractor-trailer. It then estimated that a side guard mandate would save about seventeen of those lives, because it counted only crashes at forty miles an hour or slower, and put the price near a billion dollars a year. The insurance industry’s crash-test institute answered that the true number was between 159 and 217 lives a year, roughly ten times the agency’s figure, and that guards it had tested stopped cars at forty. The advisory committee Congress created recommended a mandate in 2024. As this is written, there is still no standard.
What all of that means in a case is that the standard is a floor, and the lawyer’s work is to measure the distance between the floor and the crash. Three questions. Was there a guard, and was it the one the trailer left the factory with, or something welded on later. What condition was it in, since a guard that took a loading-dock hit last year and stayed bent is a guard that fails at fifteen miles an hour instead of thirty-five. And what did the crash actually ask of it, which is a reconstruction question: the closing speed, the overlap, the height of the car, the crush on the car’s roof and pillars showing exactly where the trailer entered. A guard that met the 1996 standard on a trailer built in 2010 is perfectly legal and is still thirty-mile-an-hour steel; the carrier that chose to keep running it made a choice, and a jury may hear what the newer guard costs.
The side is harder and still worth the fight. No rule required the guard, so its absence is not negligence by itself. But the maker of the trailer chose a design, and whether a design that lets a car slide under at thirty miles an hour is a defective one is a question a Texas jury can answer in a products case, with the agency’s own death count and the institute’s test results as the evidence. The company that could have bought the guard for a few thousand dollars a trailer is a fact too.
One word, then. Guard is what the regulations call a bar tested in a laboratory at thirty-five miles an hour, against a car hitting it squarely, on the back of the trailer only. Everything the road does beyond that is the case.
The guard’s placement: 49 C.F.R. § 571.224 (FMVSS No. 224); its strength: 49 C.F.R. § 571.223 (FMVSS No. 223); the carrier’s duty on the road: 49 C.F.R. § 393.86. The 2022 upgrade: NHTSA final rule, Rear Impact Guards, Rear Impact Protection, 87 Fed. Reg. 42339 (July 15, 2022). The corner-hit petition: 89 Fed. Reg. 53505 (June 27, 2024). The side-guard numbers: NHTSA, Side Underride Guards, 88 Fed. Reg. 24535 (Apr. 21, 2023), and the response from the Insurance Institute for Highway Safety (June 13, 2023). The physics of what happens without a guard is in Underride. General information, not legal advice.
