In the realm of accident reconstruction, and with the advent of the passenger vehicle event data recorder (EDR), colloquially known as the “black box,” comes a professional responsibility to correlate extracted data to physical roadway and vehicle evidence. Modern vehicles are equipped with various modules whose primary function is to control the various safety restraint systems, some of which can also record event data from a collision.
The present case study involves a pickup truck that brakes up a cinder rock surface with a +1.5% slope for 103.1 ft., travels up a +40.6% (22.1°) berm, becomes airborne, and collides with the ground after a horizontal distance of 45.6 ft. with a 4 ft. vertical drop, and comes to rest after 22.6 ft. (See travel profile below).
A general visual inspection of the vehicle revealed damage to the front bumper as well as the undercarriage and suspension components. Both driver’s side tires were flat and debeaded, with the front left knuckle showing disengagement from the lower control arm. Both frontal airbags had deployed, as shown below.
Reconstructing the accident backwards in time, from final rest to the beginning of the tire marks, the vehicle’s movement can be divided into four segments: (1) ground contact after becoming airborne and reaching its final rest position; (2) takeoff from atop the berm to ground contact; (3) toe of the berm to takeoff atop the berm; and (4) beginning of tire marks leading to toe of the berm.
Speeds can be approximated by dividing the vehicle movement during each phase. Physics equations for a “vaulting” motion provide an approximate speed of 28 MPH at takeoff from atop the berm (2). Using a range for coefficient of friction values for the cinder rock, and accounting for surface grades, the calculated speeds (in MPH) are as follows: 34-35 at the toe of the berm (3); and 57-63 at the beginning of the tire marks (4).
As seen below, a Crash Data Retrieval report (CDR) of the vehicle’s airbag control module (ACM) revealed that two overlapping events, one deployment and one non-deployment, had been recorded in relation to this crash, separated by 1.76 seconds.
- Event Record 1 was triggered by a maximum change in velocity of -17 MPH in 0.294 seconds (approximately -2.63 g’s)
- Event Record 2 was triggered by a maximum change in velocity of -7 MPH in 0.174 seconds (approximately -1.84 g’s)
It is likely that the first event was triggered shortly before takeoff atop the berm, as evidenced by the drop in speed from 64 MPH to 24 MPH leading up to time 0.5 of Event Record 1. Time 0.5 of Event Record 2 was likely at ground contact after the vehicle became airborne, or soon after as it was headed to its final rest position.
While a powerful resource to the accident reconstructionist, and admissible evidence in a court of law, information from EDRs must be analyzed with a careful eye and, most importantly, must be correlated to the observed road and vehicle evidence.
Written by Luis C. Flores, P.E., ACTAR.