Interpreting black box data

When a truck and a van collided at an intersection, MEA Forensic was called upon to investigate.

The Event:

A van T-boned a pickup truck crossing the street, causing the pickup to flip over. MEA Forensic was asked to figure out what happened: did the pickup stop for its stop sign? Was the van speeding, and, if so, would the pickup have made it safely across the street had the van been traveling at the speed limit?

Our Analysis:

First, our expert examined both vehicles and the scene of the crash. We used photographs taken by first responders to locate where the vehicles came to rest (Figure 1). Our team used traditional engineering methods to calculate impact speeds: the van was travelling near the speed limit, and the pickup’s speed was reasonable for a vehicle leaving the stop sign.

But our analysis went further. Our expert downloaded an electronic history of the vehicles’ travels from their airbag controllers (the “black boxes”). While physics showed the vehicle speeds at impact, the electronic records provided insight into the speeds before impact.

 Seconds
to impact
 Van speed
in mph (km/h)
 Van driver
action
 Pickup speed
in mph (km/h)
 Pickup driver
action
 5 45 (72) – 32 (52) Braking
 4 44 (70) – 27 (44) Braking
 3 43 (69) – 23 (47) Braking
 2  43(69)  – 19 (30) –
 1 40 (64) Braking 19 (30) 17% throttle
 Impact No data No data 21 (33) 100% throttle

Electronic pre-crash data. The van was traveling about 10 km/h above the speed limit and braked right before impact. The pickup never slowed below 30 km/h and had the gas fully applied at impact.

The speeds recorded in the electronic crash data let us calculate the vehicle positions for 5 seconds before the crash. The pickup didn’t stop for the stop sign and, apparently, the driver realized their mistake at the last second and tried to accelerate out of the way of the oncoming van. The van driver, on the other hand, was traveling about 10 km/h faster than the speed limit and braked just before impact. Accounting for reaction time, the van driver probably saw that the pickup wasn’t stopping as the pickup approached the stop sign.

We calculated what would have happened had the van been traveling at the speed limit when its driver saw that there was a problem. Because the pickup was accelerating at impact, a 6 mph (10 km/h) reduction in the van’s initial speed would have allowed the pickup to avoid the collision.

An overhead depiction of a vehicle collision. MEA Forensic worked to reconstruct this accident.

An overhead depiction of a vehicle collision. MEA Forensic worked to reconstruct this accident.

White pickup and red van (top) show positions when the van driver realized that the pickup was going to ignore the stop sign and the resulting impact.  White pickup and blue van (bottom) show that the pickup would have made it through the intersection if the van had been traveling 10 km/h (6 mph) slower.

The Results:

Our work told our client what happened: the pickup did not stop for the stop sign. Even though the van with the right of way was within 6 mph (10 km/h) of the speed limit, removing that excess speed would have allowed the pickup to get across the road without impact.

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.

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