In the US, cops use radars to detect vehicles that are over speeding. They use something like whistler radar to track down these law breakers. They can use the result of these radars as evidence just in case the suspect will try to say he or she was not over speeding.
Anyway, I just wish that our cops here would use such gadgets. I am sure it would be a useful tool for them to put law breakers to jail. Though buying such gadgets would entail big government funds, I guess it is worth the investment, right?