Underride: The Truck Crash We Already Know How to Prevent

 

Engineers figured out how to stop these decades ago. They keep happening anyway.

There is a kind of truck crash that is worse than the physics of an ordinary collision, and once you understand it, you notice trailers differently on the highway. It is called underride, and it is exactly what it sounds like.

In a normal crash, a car's bumper hits the other vehicle's bumper. The frame, the crumple zones, and the airbags all do their job at roughly the height they were designed for. Underride throws that out. The car slides under the trailer instead of stopping against it, and the impact lands where the windshield and roofline are, not where the bumper is. The parts of the car built to protect people never get the chance.

Rear guards help. Side guards barely exist.

Federal rules require trailers to carry a rear underride guard, the steel bar you see hanging beneath the back of a trailer. Done right, it stops a car before it slides under. The trouble is what happens on the sides. There is no federal mandate for side underride guards on most trailers, despite years of safety researchers pushing for them, and a car that hits the flank of a trailer at an intersection has nothing to catch it. I get into the mechanics of both in this breakdown of underride and override crashes.

Why the injuries are so severe

Because the impact hits above the beltline, the safety systems in a modern car are close to useless. Seatbelts and airbags are built around a crash pulse that starts at the bumper. When the trailer bed meets the roof pillars instead, the result is head, neck, and upper-body trauma of a kind you rarely see in a standard rear-ender. These are the crashes that turn into fatalities more often than any other type, which is a hard thing to write and a harder thing to sit across from a family and explain.

Being seen in the dark

A lot of underride crashes happen at night, and a lot of them come down to whether the driver could see the trailer at all. Trailers are required to carry reflective conspicuity tape and working marker lights so an approaching driver can judge where the trailer is and how it is sitting. When that tape is missing, filthy, or peeling, or when the lights are burned out, a trailer stopped or crossing a road at night can be nearly invisible until it is too late.

What goes wrong, and who answers for it

So the investigation is not only about how fast the car was going. It is about the guard: was it there, was it bent from a prior hit, was it mounted at a legal height. It is about the tape and the lights. It is about the maintenance records that show whether the carrier knew and did nothing. A crash that looks like the car's fault at first glance often turns on the condition of the tractor-trailer and the choices the company made about it.

None of this is exotic engineering. We have known how to make trailers safer for a long time. The frustrating part of underride is that the fixes exist, and the crashes keep happening while we argue about who has to pay for them.

Part of the Massachusetts Truck Safety Report.