Almost everything that proves what a truck driver was doing before a crash is held by the trucking company, and almost all of it is on a clock that has nothing to do with your right to sue.
Driver logs. Federal law requires a motor carrier to keep a driver's record of duty status — the hours-of-service record — for 6 months. After that, destroying it is lawful. That record is how you find out whether the driver had been at the wheel for eleven hours or for sixteen.
Camera footage. Forward-facing and driver-facing video usually sits on a rolling loop. Thirty days is common. Some fleets keep a week. Unless somebody asks for it, the file that shows the truck drifting across the line is overwritten by ordinary operation.
Engine data. The engine control module records speed, throttle and braking in the seconds before impact. There is no retention rule at all. It goes when the tractor is repaired, when the module is re-flashed, when the truck is sold, or when it simply goes back into service.
None of this is a conspiracy. It is a filing cabinet being emptied on schedule. But it produces a situation that almost nobody warns people about: in a state that gives you three years to file, you can be two years and nine months inside your legal deadline and have nothing left to prove your case with.
There is one thing that stops it. A preservation letter — a spoliation letter — sent to the carrier and its insurer puts them on notice that specific categories of evidence are relevant to a claim. Once that letter lands, destroying the material stops being routine and starts being a problem for them. Until it lands, the clocks keep running.