the effect on vehicle speeds of a rural motorway matrix signal, displaying a 50 mile/h advisory speed limit, was measured on m4 in both wet and dry weather. when the speed limit was displayed the mean car speed at the test site reduced from 72.1 mile/h to 67.2 mile/h in dry weather, and from 70.6 mile/h to 67.9 mile/h in wet weather. when corrected to allow for speed changes at the control site, these reductions correspond to 4.8 per cent and 4.9 per cent respectively. heavy vehicles were little affected by the signal - their normal speed was not much higher than the advisory limit. the average speed of vehicles was between 5 mile/h and 11 mile/h higher than those measured in a similar trial on m1. furthermore, speed reductions in response to the signal were much smaller on m4. these differences were mainly due to a slight difference in gradient and the higher flow of m1. a modified signal, giving drivers more information than at present, is being tested in an attempt to improve drivers' response.(a)

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