This paper introduces the operating principle and the experimental result of a vehicular gap measurement system using a pattern recognition technique. This measurement system consists of a sign board at the back of a preceding-car, a TV camera mounted on the hood of a following-car and a processing device installed in the following-car.
This sign board has a specific pattern most unlikely seen in a highway traffic environment. Utilizing the fact that the size of the image of the specific pattern taken by the TV camera is inversely proportional to the gap between the TV camera and the sign board, the vehicular gap is measured. But the image is converted into a video signal, the pattern recognition technique is applied to this video signal in order to select the specific pattern from among other visual patterns.
The pattern recognition process is performed by the processing device. This device consists of a preprocessing circuit, a pulse spacing measuring circuit, a comparing circuit, a judging circuit and a converting circuit. These circuits operate as follows.
(1) In the preprocessing circuit, the changing points between a bright spot and a dark spot in the video signal are extrated in pulse signals.
(2) The pulse spacing measuring circuit computes the time interval between the above extracted pulse signals.
(3) The comparing circuit compares the ratio of the above pulse spacing with that to be expected from the given specific pattern.
(4) The judging circuit selects only the pulse train corresponding to that of the specific pattern.
(5) The converting circuit converts the pulse spacing of the above pulse train into the distance of the vehicular gap.
This measurement system was successfully tested on Chuodo highway and Shin-ohme street. The TV camera with a 150mm lens was used on Chuodo highway so that it was possible to measure vehicular gaps ranging from approximately 10 to 160m with a high vehicular speed up to 80km/h. And using a 50mm lens on Shin-ohme street, it was possible to measure vehicular gaps 3 to 50m long.

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