Short answer: Commercial trucks carry an electronic control module that records speed, throttle, braking, engine RPM and fault codes in the seconds before a crash. That data can decisively establish speed and whether the driver braked. It can also be overwritten, lost when the tractor is repaired, or simply not preserved unless someone demands it in writing within days.
What the ECM actually captures
- Vehicle speed in the seconds before impact.
- Throttle position and whether the accelerator was applied.
- Brake application and timing — whether the driver braked at all.
- Engine RPM and cruise control status.
- Hard braking events and fault codes.
- On many units, a last-stop record and sudden-deceleration snapshots.
Combined with electronic logging device data, dispatch records and cell phone records, this builds a factual picture that testimony cannot contradict.
Why it disappears
Several ordinary things destroy it, none of which require bad faith:
- ECMs on many units overwrite older data as the vehicle continues operating.
- Repair often involves replacing or reflashing modules.
- A tractor returned to service keeps accumulating data over the relevant window.
- A total loss may be sold at salvage and the module removed or scrapped.
The truck belongs to the defendant. Nobody on their side has an obligation to preserve anything until they are told to.
The spoliation letter
This is the practical mechanism. A written preservation demand sent to the carrier and its insurer identifies the specific categories — ECM data, ELD records, dashcam footage, driver qualification file, inspection reports, dispatch records — and puts them on notice that destruction after receipt may support a spoliation finding. It needs to go out in days, not months. It is the single most time-sensitive step in a truck case.
What the data proves in practice
A driver who states he was travelling at the limit and braked hard is contradicted by a module showing 68 mph with no brake application in the final four seconds. A carrier arguing the crash was unavoidable is answered by data showing the driver never lifted off the throttle. This evidence is often what converts a disputed-liability case into a clear one — which now matters more than ever, because under the 51% bar a fault dispute is no longer just about how much.
Frequently asked questions
Do all commercial trucks have this?
Most modern heavy trucks have an ECM capable of recording crash-relevant data, though the amount retained varies by engine manufacturer and configuration.
Can I get the data myself?
Downloading it requires specialized equipment and access to the vehicle, which the carrier controls. It is typically obtained through a preservation demand followed by an inspection protocol, sometimes under court order.
What if the truck has already been repaired?
Data may still be recoverable, and the repair itself becomes relevant. If a preservation demand was received and the vehicle was repaired anyway, that supports a spoliation argument.
Does this apply to smaller commercial vehicles?
Delivery vans and box trucks increasingly carry telematics and camera systems. The category of evidence is similar even where the federal regulations differ.
Talk to a Louisiana injury attorney
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David D. Kervin, Jr.. We represent people across St. Tammany, Tangipahoa and Washington Parishes.
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Related reading
- Louisiana’s New 51% Comparative Fault Rule: What Changed on January 1, 2026
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Reviewed by David Kervin, attorney licensed in Louisiana · Last updated