Vehicles come in all shapes and sizes these days, and they also come with a wide variety of safety features. Semi trucks have a lot of those same safety features and a few that are specifically for the protection of people in smaller vehicles.
One of these that safety experts have recently deemed necessary is underride guards.
Why all trailers should have them
The high ground clearance of a tractor trailer puts the bottom of the trailer about eye level for many people in passenger vehicles. What happens if the truck jackknifes and the trailer swings around? What if the truck stops suddenly and a vehicle runs into the back of it?
In crash tests with a standard trailer and a midsize sedan, the forward motion of the smaller vehicle carries it under the trailer, which destroys the entire top portion of the car. Most real-life accidents of this nature result in fatalities.
What underride guards are
The bar beneath the bottom of the trailer in the back is a rear underride guard. In a crash, a smaller vehicle should come in contact with it in such a way as to stop the vehicle, activating the crumple zones and the air bags. Current law requires rear underride guards, but the standards are old, and safety experts and lawmakers agree that these need an overhaul to make them safer. In crash tests, many of these barriers were not effective.
Side underride guards extend across the open space between the wheels on the trailer's sides. They look like aerodynamic panels and are strong enough to catch the vehicle before it slides under the trailer. Several companies already manufacture these, so motorists may have seen them on the roadways. However, some trailers have fiberglass panels to reduce wind resistance, and these provide no crash protection.
Side underride guards are not yet mandatory, but there has been a bipartisan effort to introduce a bill, the Stop Underrides Act of 2017, and present it to Congress. Most recently, the bill has gone to a Senate committee for review.