So I read about a scientist who wrote a physics proof to prove that he did not run a stop sign, basing his argument on the fact that angular speed of a car moving near a stop sign )as observed by a distant perpendicular observer) peaks shortly before and after the stop, where the observed angular speed is zero. During that brief moment that the speed is zero, another car was blocking the sight of the officer. It's probably easier to understand if you read the actual proof. The proof is here: http://arxiv.org/pdf/1204.0162v1.pdf So I understand everything up to part IV, where he calculates the times that obstruction starts and ends. What I don't understand is why he added the two lengths together for one and subtracted them for the other. I think the problem is that he doesn't really specify where the obstructing car is at a particular time, though it could just be me because I'm not really good at mechanics. Of course, his overall argument is pretty unrealistic since it contends that acceleration starts immediately after deceleration, but I still want to understand the basis of the "proof." EDIT: I also see an issue with how he defines where his car is. From his part about speed, we can assume that he is defining x as the distance from his front bumper to the stop line. If that is the case, then it would not make sense to calculate when his front bumper is the sum of the lengths away from the line, since if partial obstruction began when he was at that point (that would make the front bumper of the obstructing car be l1 away from the line), his car would be exposed at the stop line (since the obstructing car does not reach that far). Could it be just that that part of his proof is totally bad?