Some cars and trucks are equipped with headway sensors that detect the distance between a vehicle and any vehicles or large objects in front of the vehicle. These sensors are used by adaptive cruise control and/or collision avoidance systems. Most existing headway sensors use a 76.5 GHz radar, but other frequencies (e.g. 24 GHz, 35 GHz and 79 GHz) are also in use. Some systems use infrared sensors instead of (or in addition to) the RADAR sensors.

There are two primary methods of measuring distance using radar. The first is known as the direct propagation method and measures the delay associated with reception of the reflected signal which can be correlated to the distance of the reflecting object as a function of the speed of light and the period or rather, the time delay in the transmission and receiving of the waves. The second method is known as the indirect propagation method or the Frequency Modulated Continuous Wave (FMCW) method. For indirect propagation, a modulated frequency is sent and received, the difference in the frequency can be used to directly determine the distance as well as the relative speed of the object.

Radar signals are very good at detecting objects that strongly reflect electromagnetic radiation (e.g. metal objects). Because they operate at wavelengths on the order of a few millimeters, automotive radar systems are pretty good at detecting objects that are several centimeters or larger. They are also good at looking through (i.e. ignoring objects that are small relative to a wavelength (e.g. the water droplets in fog).