An antenna (26) projects a detection zone (28) across a non-intersection portion (16) of a road (10). A scanning receiver (32) couples to and is controlled by a data logging computer (34). This computer (34) commands the receiver (32) to look for one FM local oscillator (LO) signal (22) that may be emitted from within the detection zone (28). If one LO signal (22) is detected, other LO signals (22) that may be detected at the receiver are ignored until the one signal is no longer detectable (90-106). An attenuator allows LO signals 22 emitted from radios (20) in the detection zone (28) at noisy frequencies to have the same likelihood of being detected as LO signals 22 emitted at less noisy frequencies. Detected LO signals (22) are ignored if they are detected for less than a minimum duration (100) or if they are detected for greater than a maximum duration (118). The second of two consecutively detected LO signals (22) is ignored if it occurs within a minimum duration (112) from the previously detected LO signal (22) and exhibits the same frequency (114) as the previously detected LO signal (22). A compiling computer (36) accumulates the data logged by the data logging computer (34) into a spread sheet (134).