The semi that hit you kept notes. A semi truck accident lawyer builds the case from that data: engine control modules, electronic logging devices and telematics record speed, braking, throttle and duty status. That evidence can prove exactly what the driver did before impact, but it's routinely overwritten within weeks unless someone sends a preservation letter.
A car wreck usually comes down to two drivers' stories and a police diagram. With a semi, the truck is a witness too. The only question is whether anybody gets its testimony before it's gone.
If you drive for a living, you already know this. The same ELD that tracks your 11 hours and the same driver-facing camera your safety department reviews are sitting in the other rig, too.
What a semi truck actually records
| Source | What it captures | How long it survives |
|---|---|---|
| Engine control module (ECM / "black box") | Speed, RPM, throttle, brake application, hard-braking events, fault codes | Can be overwritten by continued operation — sometimes days |
| Electronic logging device (ELD) | Duty status, driving hours, engine hours, location at intervals | Six-month federal retention floor for records of duty status |
| Fleet telematics | GPS breadcrumbs, speeding and harsh-event alerts, route history | Varies by vendor contract; often 90 days to a year |
| Forward and driver-facing cameras | Video of the seconds before and after impact | Frequently 30-day loops unless an event flag preserves the clip |
| Maintenance and inspection files | Repairs, deferred defects, driver vehicle inspection reports | Retention varies by record type under 49 CFR Part 396 |
Why the retention clock is the real deadline
Your state's statute of limitations may give you two or three years to file. That's the outer legal limit. It isn't the deadline that decides your case. The one that matters is whichever retention cycle runs out first.
A tractor that's back under a load the week after the crash keeps writing over its own ECM memory. A dash cam clip nobody flagged rolls off the loop in a month. Driver logs hit their six-month floor and can be purged.
None of that takes bad faith. It's just what happens when nobody has told the carrier to stop.
What a preservation letter does
- Puts the carrier and its insurer on formal notice that identified evidence is relevant to a claim
- Names the specific items — ECM download, ELD records, camera footage, maintenance file, dispatch communications, driver qualification file
- Makes subsequent destruction sanctionable, and in many states supports an adverse-inference jury instruction
- Is most effective in the first days after a crash, before retention cycles lapse
The other side moved first
Big carriers keep rapid response teams on call. Within hours of a bad crash, their investigators and defense lawyers can be on the shoulder photographing skid marks, talking to witnesses and downloading the ECM on their own terms.
That's not misconduct. It's competent claims handling. But it means that by the time you're out of the hospital, the defense may already have the full picture and you may have nothing.
How the data gets used
Reconstruction is where these records pay off. ECM speed data lined up against the posted limit and the curve of the road can show a driver never touched the brakes.
ELD entries checked against fuel receipts, toll transponder records and dispatch messages can expose falsified logs. That pattern supports a fatigue claim and a direct negligence claim against the carrier that pushed the schedule.
Maintenance files do the same job in brake failure and tire blowout cases. A defect written up on a driver inspection report and never fixed turns a mechanical failure into documented carrier negligence.
Act while the data still exists
If a semi was involved, the most time-sensitive thing you can do is get a preservation letter out. A free case review takes about a minute and costs nothing. For how each type of record is regulated, see FMCSA regulations & evidence.
Related guides
- 18-wheeler accident lawyer — the federal regulatory side
- Tractor-trailer accident lawyer — split tractor/trailer ownership
- What to do after a truck accident
- Who can be held liable
Sources
- 49 CFR § 395.8 — Driver's record of duty status — Electronic Code of Federal Regulations
- Electronic Logging Devices — rule and technical specifications — Federal Motor Carrier Safety Administration
- 49 CFR Part 396 — Inspection, repair, and maintenance — Electronic Code of Federal Regulations
Federal regulations and crash statistics are updated periodically. Figures cited on this page reflect the referenced publications at the time of writing; check the source for the current edition.