Black Box & ELD Data.

The driver will say the light was green and the speed was safe. The truck keeps its own record, and it does not take sides.

Modern trucks are rolling data recorders. The engine control module logs speed, throttle, and hard braking in the moments around a crash. The electronic logging device records the driver's hours. Together with dashcam footage, telematics, and the maintenance and driver qualification files, this data can reconstruct exactly what happened and whether the rules were followed.

The catch is that much of it is controlled by the trucking company and much of it is overwritten or discarded on a schedule. Once a truck is repaired or returned to service, key data can be gone. That is why a formal preservation letter, sent in the first days after a crash, is often the single most important step a lawyer takes. It puts the company on notice that destroying the evidence carries its own consequences.