Traffic officers sometimes employ an electronic device which can detect an operating radar detector in your car. The device has a sensitive radio receiver that picks up a tell-tale signal generated by the radar detector. Although most non-commercial drivers need not worry, states prohibit radar detectors in some circumstances.
A radar detector is an electronic gadget about the size of a large cell phone. Mounted on a car's dash or windshield, it picks up radio signals that police radar traffic guns produce. The detector beeps and flashes a warning light in the presence of radar, alerting you that an officer is checking your speed. It is sensitive enough to warn you before the radar gun makes an accurate speed reading; this gives you a few seconds to slow down and avoid a ticket.
Radar detectors employ a circuit called a local oscillator to tune into radar gun activity. The local oscillator operates at the same frequency range as police radar guns, producing a tiny signal of its own; radio technicians call this "leakage" because it's unintended. Because radar signals are relatively uncommon compared to AM and FM stations, the leakage signal becomes a signature that the right equipment can detect easily. Premium radar detectors have shielding to reduce the leakage, but some of the signal may escape regardless.
Electronics manufacturers sell radar detector detectors to police departments; officers use these devices to monitor traffic for the illegal use of radar detectors. The equipment picks up the signal from the detector's local oscillator and alerts the officer, who determines if the detector is legal or not.
In 2013, the state of Virginia and the District of Columbia both outlaw the use of radar detectors; federal law also forbids them for operators of commercial trucks over 10,000 pounds. Other states have laws prohibiting anything on your windshield that obstructs your view.
