The Negotiator said:
They manage it on police cars which move so there will be a system which allows it to be accurate
Furthermore, those cameras in police cameras work by measuring the distance between two points and are usually manually operated while the car is stationary.
The vids you see on Police Camera Action etc. don't measure the offenders speed, but the police cars speed and then the police simply come to the conclusion that you were also doing that speed.
A friend of mine was seen by a Sationary police car from a bridge, no speed detection equipment set up, they didn't like the sound of his xhaust so they left the post and chased him, they caught him and said he was doing over 100mph and he wasn't going anywhere near.
Their response, "well we had to go that fast to catch you so you must have been"
On top of that if the Helmet cam reads low readings for the majority of the day and one head bob causes one miscallaneous reading its more likely the police will pull you over, impound your vehilce and have it stripped down to look for a Radar scambler of some sort and have it for several days.. then not even put your car back together properly.