Mathematical Error Leads to Vehicular Homicide Charges

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A pickup truck was traveling behind a sedan approaching an uncontrolled T-intersection.  As the sedan slowed in preparation to make a left-hand turn at the intersection, the pickup truck moved into the oncoming lane and attempted to pass the slowing sedan on the left.  When the sedan crossed the oncoming lane to turn left, the front of the pickup truck struck the driver side of the sedan. 

The police completed a reconstruction of the collision and determined that the pickup truck had initially been traveling 72 miles per hour in a 50 miles per hour speed limit.  The operator of the pickup truck was subsequently charged with homicide by vehicle but insisted to his attorney that he was not traveling at an excessive speed.  The calculations performed by police were then provided to DJS Associates, Inc. for analysis.

A proper collision reconstruction, and evaluation thereof, was performed in reverse chronological order beginning with analysis of the vehicle’s points of rest and stepping backwards through the impact phase to when one of the vehicles first presented a hazard to the other.

Post-impact and impact: Based on the impact angle and how far the vehicles traveled to their points of rest, the police applied the conservation of momentum and found that the impact speed of the pickup truck was 40 miles per hour.  This was correct.

Pre-impact braking: The police documented 60-foot-long pre-impact tire marks left by the pickup.  They applied the skidding formula to determine that the pickup truck was traveling approximately 53 miles per hour at the start of the skid marks when the driver first applied the brakes.  This was also correct and should have led to the conclusion that the pickup truck had initially been traveling 53 miles per hour— three miles per hour above the speed limit. 

PRT: Then the police evaluated the driver’s perception-plus-reaction time (PRT).  PRT is the time that elapses from perception of a hazard to the start of vehicle control for tactical or strategic operations [Fricke, L. (2010)]. The PRT interval ends when the foot lands on the brake pedal or the hands begin to turn the steering wheel [Krauss, D. (2015)]. The vehicle will remain at a constant speed during the operator’s PRT as the brakes have not yet been applied.

The mistake: For an unknown reason, the police proceeded to incorrectly apply the skidding formula, for a second time, as if the operator had been braking (rather than traveling a constant speed) during his perception-plus-reaction time.  This mistake artificially inflated the truck’s initial speed to 72 miles per hour.  The police reported that the truck was exceeding the speed limit by 22 miles per hour, rather than three miles per hour, leading to criminal charges being filed against the 18-year-old driver.

The outcome: Cross-examination questions for the officer, which stepped through these mathematical errors, were provided to our client.  The judge dismissed the criminal charges at the hearing. 

1 – Fricke, L. (2010). Book Title: Traffic Crash Reconstruction. Chapter 7: Perception and Reaction in Traffic Crashes.  Page 187

2 – Krauss, D. (2015). Book Title: Forensic Aspects of Driver Perception and Response, Fourth Edition.  Chapter 18: Driver Perception-Response Time.  Page 237