We occasionally run across online postings and get questions about whether radar-based cruise control and collision prevention systems being rolled out in new cars will set off radar detectors. Car radar systems work just like police radar guns, sending out a radio signal and detecting how that signal gets bounced back. Both radar systems use Doppler shift to figure out the speed of the vehicles in the path of the radio signal. Car radar systems use the data to match your speed to the car in front, while the police radar can get you a ticket. Because of this similarity, many people assume a radar detector, designed to let you know when you are under radar surveillance, will get a false positive from a car's radar system.
To settle this question, we took two cars out on the road, a Mercedes-Benz CL550 with the Distronic Plus adaptive cruise control system, and a Mazda6 with a Beltronics Pro RX65 radar detector. Our radar detector sounds a warning when it receives both X and K band signals, the latter being the most widely used by police. The CL550 has a millimeter wave radar system that detects the speed of the car in front of it.
On our way out to run this test, we got proof that the radar detector was working properly, as it alerted us to a K-band signal. Once we got on the freeway, the driver of the Mazda6 got his speed up to 65 mph, and the CL550 driver set the adaptive cruise control to 81 mph. The CL550 matched speeds with the Mazda6 using its built-in radar. Over about 5 miles of driving, the radar detector remained silent, showing that adaptive cruise control won't set off a radar detector.
Obviously, the frequency of the car's radar is different from that of a police radar gun. Other car manufacturers with adaptive cruise controls might use different frequencies in their radar systems, but none are likely to use a frequency that could potentially interfere with police equipment.