A radar detector is an electronic device used by motorists to detect if their speed is being monitored by police or law enforcement using a radar gun. Most radar detectors are used so the driver can reduce the car's speed before being ticketed for speeding. Only doppler radar-based devices can be detected — other speed measuring devices including those using ANPR, piezo sensors, and VASCAR technology cannot be detected. LIDAR devices require a different type of sensor, although many modern detectors include LIDAR sensors. Most of today's radar detectors detect signals across a variety of wavelength bands: usually X, K, and Ka. In Europe the Ku band is common as well.