A system for detecting and counting entry and exit of passengers. Transmitted high frequency sound beams define at least two zones through which passengers pass in sequence when entering or leaving a vehicle; the motion of a passenger through the zones produces signals at a receiver which are fed to logic circuits to produce appropriate counts. The logic circuits discriminate between moving human targets and other moving objects within the zones, and include range discrimination to prevent interference from outside the detection area. In one embodiment the sensor is located above the entry area to detect passengers' heads, while another embodiment is located at a level to detect passenger foot motion.