How vital crash data can be retrieved

from vehicles

Modern cars are filled with electronic modules and sensors. These modules and sensors are constantly sending data to each other to improve the safety and performance of the car. Some of these modules store information that could be useful in determining the cause of a crash but only in recent changes to legislation in both the EU and US have manufacturers had to make this data accessible to specialist analysts. This legislation is not currently on the UK Statue books, but a large number of manufacturers sell vehicles that are very similar to those sold elsewhere in the world and these vehicles data can be accessed in the UK.

The vast majority of new cars are fitted with safety systems that are designed to activate when a collision occurs. These safety systems usually including airbags and seatbelt pre-tensioning devices are typically referred to as 'Supplementary Restraint Systems'. The SRS is controlled by a Central Airbag Module (ACM) that monitors a number of related sensors within the vehicle. These sensors include accelerometers, impact sensors, side door pressure sensors, and seat occupancy sensors. When the requisite “threshold” has been reached or exceeded, the ACM will trigger the ignition of a gas generator propellant to rapidly inflate the air bag. This is considered a deployment event. Occasionally an event will be monitored that will not meet the threshold criteria and this is considered an non-deployment event.

Modern ACMs will record deployment events, and they may also record non-deployment events. If the airbag has deployed in the crash, there will be data stored on the vehicle, and even if there has been no deployment there is still a chance a non-deployment event was recorded. Non-deployment events are stored in a piece of internal memory that is often overwritten, so the ACM would have to be examined shortly after the incident before the data is lost.

Depending on the specific vehicle, the ACM can record valuable pre-crash data such as vehicle speed, brake application, throttle application and seat-belt usage among other parameters (even if the airbags were not deployed in the accident). The ACM saves the last five seconds of data from the various modules around the car and writes it to an Electrically Erasable Programmable Read-Only Memory chip. In the event of an accident this data can be downloaded by a specialist(a list of experts is available here using the Bosch CDR tool, via a Diagnostic Link connector or if the damage to the car is significant directly from the ACM.

While the recording of such pertinent information provides another tool for accident investigators, it is the interpretation and understanding of the recorded data which truly lends value to the investigation and reconstruction of an accident. Airbag systems are not infallible, and the recorded data must be verified against other physical evidence such as skid marks, crush damage and witness marks on the seat-belt webbing.

The first use of ACM evidence in the UK was in 2006 at Birmingham Crown Court during the trial of a 21 year old man, who crashed a Range Rover Sport into a Jeep. The accident left a baby girl paralyzed. The ACM evidence allowed forensic investigators to determine the driver was speeding at 72 mph in a 30 mph zone.

In 2010 the driver of a Chrysler 300 was prosecuted at Oxford Crown Court for causing Death by Dangerous Driving. Forensic investigators were able to prove that the car was travelling at 74 mph 5 seconds before the collision and the driver only applied the brakes 0.3 seconds before impact with a line of traffic on the M40.

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This document is for information purposes only and does not purport to represent legal advice.  
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