a system that has been used in the past on known static positions is for the officer to have 2 prominent landmarks in view, as the vehicle passes one landmark he starts counting, as the vehicle passes a second landmark he stops. Times under a pre equated figure=ok
times over it and you are probably speeding.
not an exact science but it lets him pull ya and can do a character assessment from there. ie: likelihood of person to speed+type of vehicle+time past landmarks=make cheque payable to Chief Constable
www.abd.org.uk