A radar detector is a small electronic device used by some drivers to detect the presence of police radar units. Ideally, the device will pick up the radar gun's signal before the driver speeds past the police officer using it. Its use can best be described as tolerated, if not strictly legal in certain jurisdictions. Most drivers who use one keep it attached to the visor or other inconspicuous area.
What a radar detector actually detects depends on the make and model of the radar gun used by law enforcement. A very basic radar gun sends out a radio signal at a specific frequency, which bounces off a targeted object (a car's body, for example) and returns to the radar gun. This signal moves at the speed of light, so a computer unit can calculate the time it takes for a signal to return. If the target object is moving towards the radar gun, the frequency becomes higher due to a phenomenon called the Doppler shift. This means a moving police vehicle can accurately calculate the speed of a car approaching it. Police do not have to remain stationary by the side of the highway to detect speeders with radar guns.
A radar detector is tuned to the same radio frequency as most radar guns, although the police may change the frequencies often. In the same sense that a car radio can pick up the specific signal on an AM or FM channel, a detector can sense an active signal on a police radar frequency before the car gets into its enforceable range. The device sends out both an audio and visual signal to the driver, who should have enough time to slow down to a legal speed. It depends heavily on a police officer's tendency to keep the radar gun turned on steadily instead of using quick pulses. If a police car comes up quietly on a speeding car before activating basic radar, the radar detector won't offer much protection.
More modern radar gun systems have abandoned the easily detectable frequencies for less obvious bandwidths. Instead of a constant broadcast, newer radar guns use quick bursts at what police call the K-band range. A sophisticated radar detector can still pick up on these quick signals if other speeders have already been clocked. Considering that an average speeding ticket can cost quite a bit of money and a significant increase in insurance rates, the price of an advanced detection device may seem worthwhile to those drivers who tend to speed.
The latest trend in speeding detection uses laser light instead of soundwaves. A system called LIDAR (LIght-based Detection And Ranging) can be set up by law enforcers to detect speeders automatically through highly-accurate laser measurements. Because the laser signals are so narrowly focused, a standard radar detector cannot sense them before arrival. Instead, a reflective signal generated by a LED (light-emitting diode) is sent out to jam the laser light. The LED matches the frequency of the laser light on the gun and includes extra electronic "noise." This causes the radar gun to produce an unintelligible reading, which cannot be used to prosecute a suspected speeder.