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Pre-Impact Motion Analysis

ESi was retained to evaluate the circumstances around an accident involving a light pickup truck and a tractor semitrailer combination. A light pickup truck was traveling towards an intersection. Perpendicular to the travel path of the pickup, a tractor semitrailer was conducting a U-turn in the apron of a roadway intersection. While the tractor semitrailer was crossing back over the travel path of the pickup truck, the pickup truck struck the side of the semi trailer.

Neither vehicles were available for inspection and the project had a small budget. ESi conducted a site inspection but no physical evidence was observed. The primary task that emerged was to evaluate the pre-impact motion and positions of the tractor semitrailer and the pickup. The intent of the time-distance-speed analysis was to provide an estimate of the coordinate pre-impact motion of the vehicles based on reasonable assumptions for evaluation of the circumstances. While straight line motion of the pickup truck was non trivial to calculate and place on a diagram, the motion of the tractor semitrailer combination was not as straight forward due to the articulated nature of the combination.

The methodology used was plotting the loci of the paths of various points on the tractor semitrailer in 2D using a spreadsheet. The tractor semitrailer dimensions were defined and the output of was placed on a CAD drawing of the accident site. Several drawings were created at each time step with the vehicles at their calculated positions. These drawings, when viewed successively, provided a stop motion style visualization of the coordinated motion of the pickup truck and tractor semitrailer combination. ESi demonstrated that the pickup truck had plenty of time to perceive, decide, and react to the crossing tractor semitrailer and bring the pickup truck to a stop.

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