[J.Ralli]

when i did my internship with Marple Township they showed me exactly how it works, usually its any landmark or line in the road that the use. all they do it flip the switch when your front tire cosses the mark nd then hit the switch when your front tire crosses the second mark. the reaction time for both start and finish are really close to each other so it doesnt even matter. alot of cops even give the benefit of doubt and start timing when your front bumper crosses and then stops when your rear bumper crosses. On top of things, the more experienced cops can tell how fast your going by eye sight which is weird. So basically the little calculator generates your speed as you cross the markings and its pretty accurate.