A vehicle monitoring system, e.g. for identifying automatically vehicles exceeding a maximum speed limit, comprises a video camera (14) for capturing a video frame at any instant; vehicle detecting means, for example a radar (13), responsive to the presence of a moving vehicle (10) to trigger the camera (14) to capture a video frame at an instant when the image of an identification mark (e.g. a bar code 11b on the number plate) would appear at a predetermined position within that video frame; a temporary frame store (16) for the captured video frame; and on-line data processing means (17) programmed to identify the image of the identification mark in the frame store and to produce in real time an output signal (18) representing uniquely the detected vehicle.