An underride accident is when a car slides underneath the back or side of a tractor-trailer. The trailer's bed sits at about windshield height, so it can bypass the car's crumple zone and airbags. Most trailers must have a rear impact guard under federal rules, and the standard was strengthened in 2022. Side guards aren't federally required.
Rear vs side underride
| Rear underride | Side underride | |
|---|---|---|
| How it happens | A car rear-ends a trailer that's stopped, slowing or poorly lit | A car hits the side of a trailer crossing or turning across its path, or changing lanes |
| Federal guard required? | Yes, on most trailers and many trucks | No |
| Key evidence | Guard condition and certification, trailer lights and reflective tape, truck speed | Visibility and conspicuity, the truck's turning maneuver, lighting |
Rear impact guard rules
- Manufacturing standards: Federal Motor Vehicle Safety Standards No. 223 and 224 set strength and design requirements for rear impact guards on new trailers. NHTSA's 2022 update requires guards to protect occupants of a car hitting the rear of a trailer at 35 mph, up from 30 mph, including crashes where only half the car overlaps the trailer.
- Maintenance and use: FMCSA's 49 CFR §393.86 requires carriers to keep rear guards in place and in good condition. A bent, rusted or missing guard is a violation and an inspection item.
- Exemptions: some vehicles, such as certain wheels-back trailers, pole trailers and special-purpose vehicles, are excluded.
Side underride guards
No federal rule currently requires side underride guards on trailers. The 2021 Infrastructure Investment and Jobs Act directed federal safety agencies to research side underride and set up an advisory committee on underride protection. Some fleets install side skirts for fuel economy, but aerodynamic skirts aren't designed to stop a car.
Why underride crashes are so dangerous
A car's safety systems are designed around hitting something at bumper height. When a trailer's bed hits the windshield and roof pillars instead, the front-end crumple zone doesn't absorb the energy and airbags may deploy too late. That's why underride crashes often cause head and neck injuries and deaths even at moderate speeds.
What to check after an underride crash
- Photograph the trailer's rear guard (or the absence of one), its condition and any labels showing the manufacturer and certification.
- Note the trailer number and the carrier's USDOT number. The trailer may belong to a different company from the tractor.
- Check the trailer's lights and red-and-white reflective tape, which is required for visibility.
- Ask for the trailer to be preserved, not repaired or scrapped. See evidence preservation letters.
- Look up the carrier's vehicle maintenance violations in the trucking company safety lookup.
Responsibility can extend beyond the driver to the carrier, the trailer owner, a maintenance company or the guard manufacturer. See who can be liable.