Reconstructing an accident with electronic crash data

When two cars collided at an intersection, MEA Forensic was called upon to investigate.

The Event:

Car A was traveling straight through an intersection when it collided with Car B, as Car B was making a left-hand turn across Car A’s path. MEA Forensic was asked to determine the circumstances of the crash and, in particular, whether Car A had been speeding. Furthermore, we were asked whether the speed of Car A could have affected the drivers’ decisions to enter the intersection and if the crash would have occurred if Car A was traveling slower.

Our Analysis:

First, our experts determined Car A’s speed based on an analysis of photographs taken at the scene of the accident and measurements of the damage to both cars. Our team used traditional engineering methods—based on energy and momentum principles—to calculate the speeds of the cars at impact. This analysis indicated that Car A was traveling close to the speed limit at the moment when the two cars collided.

But these methods did not tell the full story. When we downloaded electronic crash data from the “black box” in Car A, we discovered that Car A had been traveling at nearly twice the speed limit as it approached the intersection. The data revealed that the driver braked for about 3-seconds before the crash but, because Car A was equipped with anti-lock brakes, it did not leave any skid marks at the scene.

to impact
 Car A’s speed
in mph (km/h) 
5  56 (90)  No
4  57 (92)  No
3  58 (93)  No
2  53 (85)  Yes
1  35 (56)  Yes


Electronic crash data downloaded from Car A recorded a speed in excess of the speed limit in the seconds before the crash. Braking was recorded starting about 3-seconds before impact, corresponding with a decrease in Car A’s speed to a value near the speed limit when the impact occurred.

From the speeds recorded in the electronic crash data, we calculated the position of Car A at the moment when the driver of Car B initiated their left-hand turn. Because of its high speed, Car A was a long way from the intersection at this time. Had Car A been traveling at the speed limit, or even as much as 50% over the speed limit, Car B would have had enough time to safely make a left-hand turn.

Using an electronic data recorder (EDR) to obtain information about a vehicle collision

Based on the driver of Car B’s statement (that they’d started their left turn when the traffic light turned amber) we also calculated that Car A was far enough from the intersection that a typical driver, approaching at a typical speed, would have stopped for the traffic light. But Car A’s actual speed was actually too high for it to stop short of the intersection.


The Results:

Our work helped our client show that Car A was speeding before the collision and describe how the driver of Car B acted reasonably given the circumstances. Furthermore, our results showed how the high speed of Car A affected the driver’s ability to stop for the changing traffic light.

Electronic crash data formed the foundation for our conclusions. Our experts routinely download and analyze this kind of evidence from vehicles involved in accidents. Almost all cars built since 2012, and many older ones, will record data about impact severity, airbag function, seatbelt use, pre-impact speeds, and driver actions. Our experts have also conducted leading-edge research into the accuracy and limitations of this kind of evidence.

If you have any questions about whether this kind of information might be available and useful in your case, do not hesitate to contact our collision reconstruction experts.

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