This EDR case study involves a head-on collision that occurred between a black sedan and a red SUV as seen in the image above. The sedan had been traveling in the inner lane of the curve, cornering to the right. Gouges near the white lane line are clear evidence that the SUV driver had crossed the centerline into the sedan's travel lane. The police investigation concluded the SUV driver was at fault and there were no witnesses to state otherwise.
The insurer of the SUV requested an investigation of the event. The roadway was mapped with a laser scanner and a drone. The location of the gouges was documented, along with the curvature of the road and the banking of the pavement. The vehicles were measured and information was downloaded from the vehicle event data recorders (EDR), which included measurements of vehicle operation for five (5) seconds leading to the collision.
The EDR data from the sedan showed the driver had the cruise control engaged and traveled at 59 mph through the curve. The sedan was a late-model domestic car with stability control and among the data recorded were lateral accelerations as the vehicle traveled through the right-hand curve.
Lateral acceleration is the measure of the force that tends to slide your body sideways in your seat while cornering. It is related to your travel speed and the amount of steering effort (curvature of path). It is typically reported on a scale that references earth's gravity. A vehicle traveling in a straight line experiences no lateral acceleration (0 G's). A performance driver in a sports car can acheive cornering lateral accelerations near 1G. In this particular corner, test data showed that a motorist traveling at 59 mph would experience approximately 0.12 G's of lateral acceleration on average.
The accompanying diagram shows the cornering accelerations recorded by the sedan approaching the crash (orange line). A dashed yellow line is included which represents average cornering accelerations collected by an instrumented car traveling through the same section of curve at 59 mph.
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The EDR data shows that five (5) seconds prior to the crash the sedan was cornering to the right with path curvature less than needed for the roadway. Three (3) seconds prior to the crash there is an apparent steering correction, followed by hard cornering for the final two (2) seconds. The data trace ultimately tells the story of an inattentive driver drifting over the centerline, regaining attention to the roadway, and then evasively steering to the right towards the inner travel lane.
With this recording of the duration and amplitude of the swerve response, the path of the sedan in the two seconds prior to the crash is defined. The data shows the sedan had been located in the oncoming travel lane before the evasive steer maneuver as shown in the following diagram.
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The EDR data from the SUV was limited and did not include steering or cornering measurements. Given what we know about the sedan's motion and the damage patterns to the vehicles, it is likely the SUV driver was in his travel lane and faced with an impending head-on collision with only a few seconds before the vehicles would meet. At this point in time, the SUV driver had no way of knowing the sedan's next moves. The SUV driver's choice at that moment to evasively steer towards the inner lane was reasonable. Unfortunately both drivers steered toward the inner lane simultaneously where they collided head-on.