Recurrent congestion on the UK motorway network can be a major cause of delay. One approach to eliminating or reducing the effects of congestion which has proved successful, both in the UK and abroad, is ramp metering. This uses traffic signals on entry slip roads to regulate the flow of traffic joining the main carriageway, with the aim of reducing disturbance to the mainstream traffic. There is a range of different approaches to ramp metering, including standard fixed time signals, traffic responsive schemes and coordinated systems, in which the signals on a number of adjacent merges are linked. This study details a comparative study of several different strategies using SISTM (Simulation of Strategies for Traffic on Motorways), a microscopic motorway simulation program which was developed at TRL. The results from this study suggest that, under certain high flow conditions, traffic responsive ramp metering could have some effect in reducing congestion. However, under other conditions, implementation of ramp metering might result overall in increased delays to traffic. When coordinated ramp metering was investigated, no discernible benefits emerged. (A)

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