For most of the history of collision investigation, the vehicle itself could tell the investigator only what its damage recorded. That is no longer true. A large and growing share of vehicles on the road carry an event data recorder, and the few seconds of data it stores around an impact are frequently the single most reliable piece of evidence in the case. They are also among the easiest to lose.
What an event data recorder stores
An event data recorder is a function, usually within the airbag control module, that captures a short window of vehicle data when a crash-like event is detected. UN Regulation No. 160 sets out what a compliant recorder must capture, and at what resolution: among the required elements are longitudinal and lateral delta-V through the crash pulse, vehicle speed, accelerator position, service brake status, engine speed, steering input where available, the status of the driver’s seat belt, and the deployment timing of restraints, recorded for the seconds leading up to the event and for the event itself. The regulation specifies sampling rates, the number of events that must be retained, and that the data must survive the crash and be retrievable with a commercially available tool.
In the European Union, Regulation (EU) 2019/2144 made an event data recorder mandatory for new vehicle types from July 2022 and for all new passenger cars and light vans registered from July 2024, by reference to the UN regulation. Whether a particular vehicle in the United Kingdom carries a compliant recorder depends on its type approval and model year; many vehicles built before those dates have recorders fitted voluntarily, with varying content, and most vehicles sold in the UK are built to the same specification as their European equivalents.
Why it matters to the analysis
Recorded delta-V, speed and brake status replace estimation with measurement. A crush-based delta-V calculated to a range of several miles per hour can be checked against a recorded value. A dispute about whether a driver braked before impact, or how long before, is answered by the brake switch trace. An argument about speed is anchored by the speed channel, with the caveat that it is a processed wheel-speed signal and the report should treat it as such. In a low-velocity impact claim, a recorded delta-V may settle the engineering question entirely.
None of this is automatic. The data has to be read by someone who understands what each field means, how it was sampled, what the recorder treats as an event, and what the known limitations of the particular system are. A recorder that did not trigger is itself informative; it usually means the crash pulse fell below the threshold at which the system records, and that is a bounded statement about severity.
The preservation problem
Event data recorders have limited memory. Many retain only a small number of events and overwrite the oldest when a new one occurs; some clear data in circumstances an investigator would not expect, such as a later minor event or a battery disconnection during repair. A vehicle sent for repair, sold for salvage or scrapped takes its data with it. In practice the data is often gone before anyone thinks to ask for it.
That makes preservation the first engineering task in any case where a recorder may exist. In civil proceedings the duty to preserve documents extends to electronic data, and Practice Direction 31B expressly contemplates data held on devices. In criminal investigations the Code of Practice under the Criminal Procedure and Investigations Act 1996 requires the investigator to retain material that may be relevant. A short letter, sent early, asking that the vehicle be retained and the airbag control module not be disturbed until a download has been carried out, costs almost nothing and may preserve the best evidence in the case.
Retrieval
Downloading the data is a controlled process. The industry-standard tool connects to the vehicle’s diagnostic port or directly to the module and produces a report in a defined format; where the vehicle’s electrical system is damaged, the module may have to be removed and read on the bench. The download should be carried out by a technician with the appropriate tool, or witnessed by someone competent, documented with photographs and a chain of custody, and disclosed to the other side with the report that relies on it. Data produced without that discipline invites argument about its provenance that the data itself cannot answer.
For those instructing
Ask three questions at the outset of any road traffic matter. Does either vehicle have an event data recorder? Where is each vehicle now, and who controls it? Has a download been carried out and, if so, by whom and with what tool? If the answers are unknown, the next step is a preservation letter, not an analysis. The analysis will be better for it.