I thought that was VASCAR,it works as follows, you pass a set point and they start a stopwatch, they pass the same point and start the distance measure, you pass the second point and they stop a stopwatch, they pass the second point and stop the distance counter (all this in the VASCAR unit - alternatively they can pre-input a distance and just start and stop the clock over the distance ) they now have time over a distance and VASCAR crunches out the speed.
I just wondered how this thing was operated. They see you pass a bridge or start???? see you pass and stop?????
Obviously parallax type errors and reaction times would make an impact. Just my suspicious mind in operation perhaps. If I was 2 mph over a ban limit and it hadn't been done with something very reliable I would be kicking and screaming all the way to court.....
I don't know the intricacies of it all, but I guess he followed me as I passed under one bridge, started his gadget and as I passed under the second bridge (exactly half a mile away), stopped it. The onboard compooper system unit then calculated the time, distance and speed according to Archimedes counter rabbit principle law of Pythagoram, this then alerted him to the fact that Iwas indeed going very, very fast!
They showed this on a past Traffic Cops programme. They are very highly trained in it, but there is some room for error. I would suspect that as you were close to the ban limit, but they couldnt prove it due to potential error in the system, and they didnt have a ticket, they let you go. As long as you werent driving dangerously, and they thought you had learnt your lesson, it most importantly saves them some paperwork.