Licensed Professional Engineers
A newsletter dedicated to keeping attorneys informed of the technical side of product liability cases.
Issue 55: Vol. 1 June/July 2013
© 2013 M.A.S.E. LLC
( 479) 549-4860
FORENSIC CLUES # 55 - "Crash Data" by John L. Ryan
Modern vehicles are equipped with event data recorders (EDR) that are a component of the safety system, normally a subcomponent of the airbag control module. These event data recorders record various information when a collision occurs. This information includes
Pre-crash vehicle data
Vehicle speed at start of crash
Change in velocity as result of crash
Seat belt status
Airbag deployment data
Additional information may be available, including lateral acceleration, whether the vehicle rolled, where damage was detected, data on multiple impacts, status of cruise control, engine rpm, seat position, steering wheel angle, yaw rate, light status, door status, window status and more.
How Can This Data Be Used?
Crash data can be used to determine crash severity, driver behavior, the status of various car systems before impact, as well as providing information that is very helpful in accident reconstruction.
A Sample Crash—What Can We Tell From the Crash Data?
Figure 1: First page of crash data
We know that there were three impacts to this vehicle in the following order:
1. Recorded Deployment level impact
2. Non-deployment level impact, not recorded
3. Recorded non-deployment level impact
How Do We Know This?
Figure 2: Shows 2 recorded events
Figure 3: Shows 1 additional event
The wording of the crash data can be confusing, but the data indicates that the unrecorded event was between the recorded events. The third impact likely overwrote the non-deployment event, since the third impact was a deployment level impact, but there were no airbags left to deploy.
The best measure of accident severity is deceleration in amount of g's. Some vehicles report crash data in g's, some use the change in velocity like this Chevrolet. The g's can be determined from simple calculations.
Figure 4: Crash Pulse Showing Delta V
On the longitudinal axis (front to back), the maximum change in velocity is 6.10 mph. This is the change in velocity over a 10 millisecond time period. To find the deceleration over this time frame, divide the change in velocity by the time, adjusting units, we determine the amount of acceleration to be 27.80 g’s.
One thing notable is that the nondeployment event is more significant than the deployment event. This was considered a nondeployment event simply because there were no more air bags to deploy. The data shows a maximum change in velocity of 8.63 mph for the nondeployment.
Also of significance is the change in direction of the acceleration. The deployment event involved forces impacting the car to the front, and on the left side, based on the sign convention (+ or -). The non-deployment event, which occurred .72 seconds later involved forces acting on the rear and the left side. This likely indicates multiple impacts occurring between the two vehicles.
Figure 5: Shows nondeployment change in velocity
Other significant information relevant to accident reconstruction:
Figure 6: Pre-crash steering wheel angle
The data from the steering wheel position seems to indicate a loss of control preceding the accident.
Based on this limited information, we can determine that there was likely a loss of control resulting in the vehicle sliding in a right curve, impacting a vehicle or object on the front and left side of the vehicle, then short thereafter being impacted again, likely by the same vehicle, based on the short time period between the two impacts.
The maximum deceleration is 39.3 g's. The total change in velocity is about 50 mph. The average deceleration over the duration of the collision 23.5 g’s.
So how severe of an accident is this?
-the total change of velocity is large
-the maximum deceleration is large at 39 g's from the rear-end impact
-the frontal impact is 18.5 g's (this is likely underreported)
-moderate to severe injuries can be expected from this collision
-driver is apparently NOT belted according to data, injuries will likely be very severe
Let's assume we have a bit more information. We learn from the police report that the driver claims that there were actually three vehicles involved. The driver of the subject vehicle is trying to say that he was hit first from behind, which caused the loss of control, and the driver of this vehicle sped off. According to the police report, there are skid marks, which after you examine realize support the theory that he was traveling too fast, lost control of the vehicle, which started the accident sequence. What of a third vehicle? We know that the first impact was to the front and left of the vehicle, disproving any possibility of a phantom rear-ending hit and run vehicle as the initiating event.
Give us a call to see how crash data can help your case. (479) 549-4860 / firstname.lastname@example.org
Please call us to discuss any questions you have about crash data retrieval or analysis. (479) 549-4860
© 2013 Mechanical and Safety Engineering
Mechanical and Safety Engineering LLC (MASE)
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Siloam Springs, AR 72761
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