When trying to determine whether there is meaningful data in an Event Data Recorder (EDR) or "Black Box" after a crash and how long the information lasts, it is important to consider the two types of events stored in the EDR.
- Non-deployment Events
- Deployment Events
In a non-deployment event the collision has produced forces heavy enough to trigger a recording. In late model vehicles with a "Delta V" (change in velocity) of 5 mph the collision force is large enough to wake up the "Black Box" and trigger a recording. If the collision was heavy enough to trigger the recording but not deploy the airbag, the event is known as a non-deployment event. In this type the data is volatile and is stored for only 250 ignition cycles which is usually about 3 to 4 weeks of normal driving activities.
The second type is a deployment event. This is where the airbag deploys. In that case the information is stored indefinitely and is available in the airbag control module (ACM) or Black Box. Even if the ACM is removed, the data is still accessible directly from the ACM. Sometimes the airbag control module may have been taken out of the vehicle for preservation and use at a later time. If this is the case, the information is then only able to be imaged directly from the EDR.
"A couple of common questions about EDR imaging are, "How do you know the information is valid?" and "If someone images the information does it still leave the data for someone else to image?""
First, the technology has been around and readily available for imaging for 20+ years and was first created for the purpose of helping vehicle manufacturers to know how their vehicles perform. The National Highway Traffic Safety Administration (NHTSA) has been studying the ramifications of having this technology and has given guidelines to manufacturers to standardize what is being recorded.
There are many different types of information available from the EDR such as Delta V (Crash Severity), Speed, Brake, Throttle, RPM's and Seatbelt Status just to name some of the information. Some vehicles show the speed at impact and others at approximately 1 second before the impact.
The data can include the speed and other information at increments up to 5 seconds before the collision. This electronic information can be verified by independent linear momentum or other types of speed calculations based on the scene data which includes the impact and final rest positions, vehicle weights and post trajectory paths etc. Crash testing during seminars and symposiums in the field of traffic crash reconstruction has validated the use of the linear momentum and various other equations for years which is then in turn used to validate the data from the crash data recorder.
The simple answer to the second question,"If someone images the information does it still leave the data for someone else to image?" is "Yes". The information is only copied and not removed from the Black Box. If the airbag control module is removed from the vehicle then the information can only be imaged directly from the ACM. If it's not removed the information can be imaged from the vehicle.