To understand radar detectors, what they're detecting has to be understood, first. Radar guns are basically radio transmitters and receivers. The radio waves' frequency changes when it hits a moving car, and the amount of change to the radio wave is consistent with the car's speed: the faster the car, the greater the change to the radio wave. The radar gun converts the change to a relative speed: the difference between the police officer's speed and the target's speed. (Notice that if the police officer isn't moving, his speed is 0 mph, so the speed revealed by radar equals the target's speed and no math is needed.)

Police also use lidar, which uses the same principles as radar with infrared laser bursts instead of radio waves. This circumvents radar detectors and is highly focused. You almost have to be the target to detect it, in which case you're already caught. Though it can be used like the old radar, lidar is more common where the ticketing system is automated, where the detection of a speeding car activates a camera to catch the car's license plate, and a ticket is mailed to the offender.

Radar can be easily detected with a radio receiver comparable to your AM/FM radio but set to the frequency range used for radar. (Other frequency ranges happen to be full of such things as television broadcasts and distance automatic car openers.)

However, such a basic radar detector means that if you happen to be the first target when the police officer activates his or her radar gun, you're caught by the time the detector tells you about it. It's passive, alerting you to the use of radar in the area and nothing more. This is often enough, since radar spreads so much that you'll usually detect it before you're the target. Lidar, by contrast, is highly focused, so you'll probably be the target before you detect it. (A black car—which absorbs more light—and plastic covers for your license plate can decrease the lidar ability to detect you while not harming your detector range, which may give you enough time to slow down before your speed is detected.)

Active detectors jam the speed readings, interfering with the signal that reveals your speed. Radar jammers and lidar jammers exist, though the police are continuously advancing the technology to detect speeders. Any jammer can become obsolete at the turn of a hat, making the investment of a few hundred dollars abruptly worthless.

Warning: radar detectors themselves give off telltale radio frequencies that are detectable with a VG2 device. Radar and lidar detectors are illegal in some areas, making their owners liable to prosecution.

The ultimate way to avoid speeding tickets is simple and widely known: don't speed. But how many drivers actually follow that method?