Crash Speed Reconstruction on Route 6
At 21:15 you are dispatched to a two-vehicle crash on a straight, level two-lane road. The striking vehicle left 132 feet of skid marks ending at the impact point. Its event data recorder shows the driver's perception-reaction time as 1.5 seconds. A witness, who was traveling at a steady 45 mph (the posted limit), says the striking vehicle passed her 396 feet before the impact point and that the pass occurred 5.5 seconds before impact. Your crash form requires the striking vehicle's speed in feet per second and in miles per hour, the reaction distance, the total distance from the driver's perception to the impact point, and a percentage comparison against the posted limit. Use 1 mile = 5,280 feet. Department policy requires supervisory review when a vehicle's speed exceeds the posted limit by 10 percent or more.
1. Compute the striking vehicle's average speed over the witness segment in feet per second and in miles per hour, showing the unit conversion.
Read the worked answer
Treat the witness segment as a moving clock: the vehicle covered a known distance in a known time.
- Average speed = distance ÷ time = 396 ft ÷ 5.5 s = 72.0 ft/s.
- Convert seconds to hours: 72.0 ft/s × 3,600 s/h = 259,200 ft/h.
- Convert feet to miles: 259,200 ÷ 5,280 = 49.09 mph.
Report 49.1 mph but carry 49.09 into the percentage step, because rounding early can flip a threshold decision. Note what this number is: an average over one 5.5-second window, not a measured impact speed.
2. Compute the reaction distance and the total distance from perception to impact. Then say whether the witness account and the physical evidence are consistent.
Read the worked answer
Reaction distance = speed × perception-reaction time = 72.0 ft/s × 1.5 s = 108 ft. During that time the vehicle is not yet braking.
Skid marks begin when braking starts, so total perception-to-impact distance = 108 + 132 = 240 ft.
Convert that to time: 240 ÷ 72.0 ≈ 3.3 seconds before impact. The witness was passed at 5.5 seconds before impact, 396 feet out. So perception began roughly 2.2 seconds and 156 feet after the pass. The two accounts fit together rather than conflicting: the pass, the perception, the braking, and the impact form a single sequence consistent with a driver who noticed the hazard late but before the skid marks start. That consistency is worth stating in the narrative, because it corroborates both the witness and the recorder.
3. Does the policy's 10 percent threshold trigger supervisory review? Explain, including any cautions about the margin.
Read the worked answer
Percentage over the limit = (49.09 − 45) ÷ 45 = 4.09 ÷ 45 = 0.0909, i.e. 9.1 percent.
The threshold is 10 percent, so review is not triggered - but by a very thin margin. Ten percent over 45 mph is 49.5 mph; the calculated speed is about 0.4 mph below that. Three cautions belong in the report:
- This is an average over one segment, not necessarily the speed at impact; a different segment could yield a different figure.
- Rounding is stable here (72 ft/s and 49.1 mph still give 9.1 percent), so the conclusion does not depend on dropped decimals - but say so, because a reader will otherwise wonder.
- Because the margin is under half a mile per hour, document the calculation and note the near-threshold result so the supervisor can decide whether other grounds warrant review.