In January 1978 the installation of ten sets of vehicle speed measuring and recording equipment was completed on a 19 kilometre section of the north-bound carriageway of the M1 between junctions 10 and 13 in Bedfordshire. Vehicle speeds were recorded 400 metres past standard post mounted matrix signals to measure the effectiveness of the signals. During the period January 1978 to November 1979 there were 421 occasions when matrix signals were set for 120 minutes or more. Accident, visibility and roadworks were the most common reasons for signal settings. In total 1576 signals were set during this period with an advisory 50 mile/h speed limit being most frequently displayed. For short vehicles an advisory speed restriction of 50 mile/h set for accidents or other incidents slowed vehicles by about 6 km/h (4 mile/h). For a 50 mile/h restriction set for poor visibility the reductions were about 8 km/h(5 mile/h). Similar reductions were measured on weekdays and weekends. Speed restrictions of 60 mile/h had less effect on speed while larger reductions were measured for 40 mile/h, 30 mile/h and restrictions showing lane closures. Long vehicles slowed by about half the reductions measured for the faster travelling short vehicles. (A)

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