A system, which measures the time-gap between successive vehicles and illuminates a warning sign if this gap is less than a pre-set level, is shown to affect the close-following behaviour of drivers on the A332 at Ascot. To test the effectiveness of the sign, measurements of inter-vehicle gaps were made at two points, 330 metres and 880 metres, 'downstream' from the sign. An improvement of about one third in the proportion of drivers following at less than 1.0 second was detected at both monitor points, after installation of the sign. There was no statistically significant reduction in this effect over the five year period of the experiment. The number of accidents was insufficient to indicate whether there had been a reduction associated with the operation of the sign. (A)

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