Most truck crashes are ordinary collisions at a larger scale. Two kinds are not, and they tend to produce the worst injuries and the most technical cases.
Underride
An underride happens when a smaller vehicle passes underneath the side or rear of a trailer, so the car’s crumple zone and airbags never engage. The passenger compartment takes the impact directly.
Two features matter:
- Rear underride guards. Trailers are required to have rear impact protection to a specified strength and height under 49 CFR 393.86. A guard that was missing, damaged, or mounted too high changes the physics of the crash — and the liability analysis.
- Side underride. There is no equivalent federal requirement for side guards on most trailers. A car that goes under the side of a trailer at speed is a different case from a rear impact, and the absence of a guard is not automatically negligence.
What this means practically: the condition of the guard and the geometry of the impact are evidence. They are inspected, photographed and measured — and they are not something a police report usually records.
Jackknife
A jackknife is the tractor folding against the trailer so the two form an angle. It usually follows:
- heavy braking, especially on a low-friction surface
- a sudden steering input
- a brake defect or an imbalance between tractor and trailer brakes
- load shift
The interesting question in a jackknife case is why the combination lost stability. That answer lives in the brake system data, the load documentation and the maintenance history — the same records that get overwritten.
Why both crash types are evidence-heavy
| What the case turns on | Where the answer lives |
|---|---|
| impact geometry | photographs, scene measurement, reconstruction |
| guard condition | inspection records, photographs of the trailer |
| brake performance | maintenance records, ECM brake data |
| load and weight distribution | bills of lading, scale tickets, dispatch |
| driver input | ECM, dashcam, ELD timeline |
None of that is in the police report. All of it belongs to the carrier.
The practical consequence
If the crash involved an underride or a jackknife, the ordinary “who hit whom” framing is not enough. The case becomes about the condition of the equipment and the condition of the load — which means the preservation demand needs to name maintenance records, brake data and load documents specifically.