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An engineer really should know better than this. At 60 mph the cop would have 15 seconds on the stop watch. At (say) 90 mph it is 10 seconds. A person can work a stopwatch to within +/- 0.2 seconds with practice, judging from autocross experience, and a little worse without. That is still accurate to within 2 mph with practice. The point is, a simple guess as to how well a person works the watch translates directly into how accurate the speed reading is. At typical speeds, the uncertainty is small. At all speeds, the error is negligible relative to the size of the violation.
- John

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