Physics Over Perception: How Evidence-Based Analysis Dispelled Eyewitness Account

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A sedan traveling straight collided with an electric bicycle attempting a left turn across its path from the same direction.  The crash occurred on a downhill, undivided two-lane road with a posted speed limit of 35 mph.  The cyclist was ejected forward, sustaining fatal injuries, while the vehicle sustained damage to its front passenger side, including a deflated tire and a leaking washer fluid reservoir.  Eyewitness accounts varied dramatically, with one observer estimating the vehicle’s speed at over twice the limit.

Without direct video of the crash, investigators faced conflicting witness statements and an incomplete police report that overlooked key physical evidence.  The official investigation relied heavily on subjective testimony, accepting the witness-estimated speed of 80 mph without any mathematical backing.  Critical details, such as a fluid trail from the vehicle, were captured in the bodycam footage of police who first responded to the crash but were not documented in reports or scene photos—the evidence had evaporated before the official investigation team arrived to photograph the scene.

Retained by the defense, the collision reconstruction engineers at DJS Associates, Inc. conducted a thorough forensic analysis, emphasizing the review of all available evidence to uncover overlooked details.

Damage Analysis: With the left-turning cyclist crossing from the driver’s right and the impact location just one foot inboard from the vehicle’s passenger side, damage analysis established that the cyclist had entered the path of the vehicle just fractions of a second prior to impact.  It was also noted that the vehicle’s front right tire was deflated and that the windshield washer fluid reservoir was punctured and empty.

Scene Evidence: Roadway markings and the cyclist’s point of rest were shown in the photographs taken by police.  Because the vehicle had left the scene prior to police arrival, a crucial element of this reconstruction was the bodycam video from the original responding officers, which clearly showed a trail of windshield washer fluid left by the vehicle.  The abrupt termination of the trail, in the form of a large pool on the roadway, defined the vehicle’s initial point of rest immediately following the crash.  This piece of evidence was not mentioned nor evaluated in any of the police reports, indicating that the bodycam footage may not have been reviewed by their investigators. 

Multi-Method Speed Calculations: Multiple physics-based models were used to calculate the impact speed of the sedan independently, and the results were cross-verified for reliability:

• Cyclist Projection Distance: Using the Searle Equation, a standard method for rider ejections, the vehicle’s impact speed was calculated based on the cyclist’s forward projection distance.  This yielded 32–38 mph, accounting for trajectory and roadway slope.
• Tire Mark Evaluation: A 200-foot long, post-impact tire mark was left by the sedan.  While 200 feet of hard braking would typically indicate a speed of 65 mph (still not the 80-mph speed estimated by a witness), this tire mark was the result of the vehicle’s deflated front tire and extended far beyond its initial point of rest—defined by the fluid pool—due to the vehicle driving forward after it had initially stopped.  Accordingly, only the portion of the tire mark between the point of impact and the fluid pool could be used to correctly calculate impact speed.
• Post-Impact Travel Distance: Evaluating the distance that the vehicle traveled from impact to rest (defined by the fluid pool) on the dry, sloped roadway and factoring in the single deflated tire, application of the laws of physics established an impact speed of 34–39 mph.

DJS’ analysis revealed the vehicle was traveling under 40 mph at impact, far below the witness’s 80-mph estimate.  Multiple independent speed calculations converged on this range, demonstrating how forensic video analysis methods can reliably determine dynamics, even without video of the crash itself.  The fluid trail, visible only in bodycam footage, proved pivotal; it signaled that the vehicle had stopped promptly post-impact and contradicted claims of an excessive 80-mph speed that the official investigators blindly accepted.

When analyzing collisions without video, the reconstructionist must identify and evaluate all available physical evidence before reaching a conclusion.  By reviewing every layer of data and using proven methods to deliver unbiased insights, DJS’ reconstruction reflected the true dynamics of the collision rather than inflated witness estimates.