The two most common speed detection methods used by police officers are radar and laser (LIDAR). Radar works by sending out radio waves, usually in pulses, and measuring their reflection. A moving object changes the frequency of the radio waves, depending on the rate of speed at which it is moving. Many police use it for measuring speed, though there have been instances where radar has been successfully challenged in court. Motorists have the right to see the radar gun because it is the basis upon which the police officer is stopping the vehicle. Furthermore, the calibration records can be subpoenaed. If the gun was not calibrated according the schedule, the changes may need to be dropped.
Laser detection (LIDAR, light detection and ranging) is very different from radar. It uses light to measure the distance between the gun and the car. This distance is measured several times and the change in distance is used to calculate the speed of the vehicle. In order for the gun to work, the light needs to reflect off some part of the vehicle, usually the license plate. Unlike conventional radar, LIDAR is not easily detected by devices used to locate or disrupt police speed-detection guns.