My brother the LEO sez… There are three types of speed control beyond the basic traffic patrol 1) Public safety where they put a cruiser in a prominent location so people can plainly see it from a great distance causing most drivers to be on good behavior and be aware that police are in the area. 2) speed enforcement where there is a visible and sometimes announced speed patrol -- see public safety 3) revenue generation/punishment speed enforcement -- concealed (hiding) and observing traffic, shooting a targeted vehicle, make the stop and reload, err, return to the hunting blind.
When performed correctly, the officer observes what he believes to be a traffic miscreant, targets that vehicle and shoots it with radar/lidar ensuring that certain conditions are met. Conditions include beam spread, angle to traffic (cosign error), target accuracy, proper equipment settings, etc. In this case when your detector goes off your speed has already been locked, your vehicle has been identified among the traffic and the officer has visually determined you are moving at a significantly different speed from the surrounding traffic. Riding with my brother in his cruiser he has demonstrated the ability to visually determine vehicle speed typically within 2 mph. He points to a car and says, 78 – trigger the gun, actual 79; points to another car and says 63, gun, actual 63. When police are targeting specific cars and use instant on radar or lidar your speed is locked before or just as your radar detector goes off.
Fortunately, the LEOs in my area tend to drive around with the radar unit on all the time which gives me the opportunity to check my speedometer to verify my speed hasn’t accidentally crept over the limit before I actually come into range
[bTW, when my brother was a patrolman his supervisor kept saying, “Officer Smith, have you lost your citation pad?” To which my brother says, “I make more than 50 stops a week.” His supervisor then replies, “Make sure that those stops result in tickets.”]
