Police are using different methods to make an initial traffic stop. 

Police Traffic Radar Speeding Ticket - Expert

One such method is  called C & C coupled with police radar for speeding violations in order to make the initial traffic stop, to establish the probable cause. This might lead to an arrest.

Police are using visual estimations, and radar to issue traffic speeding tickets for probable cause / reasonable articulate suspicion, based on articulate facts.

This video show a complete cameo police vehicle with a radar unit, antennas, plate scanners, and radio antennas.  At some point in time, the first, police officer makes a visual estimation, activates his radar device.  The lead police vehicle radios to the second police vehicle,  to make the traffic stop.   Shortly, there after, the police officer / trooper actives his emergency lights and makes the traffic stop. 

Police are experts in traffic speed measuring devices, retain your own expert for the highest rate of success.