Ramp Metering
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Ramp Metering is a system which controls the rate at which vehicles join a motorway by using traffic signals on the entry slip (or "ramp"). By allowing traffic to join the main carriageway at a measured (or "metered") rate, the system aims to delay the onset and/or duration of flow breakdown that leads to congestion. Peek, in conjunction with IPL, is developing the latest generation of ramp metering systems for the Highways Agency. The system comprises a Ramp Metering Controller (RMC) co-located in a type 600 cabinet with a Traffic Signal Controller, connected to a set of signal heads at the end of each ramp. |
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The RMC is connected to queue, presence and release loops on the ramp, and receives vehicle data from MIDAS Outstations on the main carriageway. It processes the ramp and MIDAS vehicle data in order to determine the traffic conditions on the ramp and on the main carriageway.
Using this processed data, the RMC runs main carriageway metering algorithms to determine the required flow rate on the ramp in order to prevent flow breakdown on the main carriageway, and queue management algorithms to determine the required ramp flow necessary to maintain the desired queue length.
Two further algorithms, Switch On/ Off and Queue Override, are also run to determine whether conditions are appropriate for ramp metering to be switched on/off and to prevent the queue on the ramp backing up on to the all purpose roads. The RMC arbitrates the outputs from the algorithms, converts the required flow rate into signal timings and sends these to the Traffic Signal Controller, which in turn drives the signal heads.
Peek will implement 30 ramp metering installations by the end of 2006. The first 10 sites, located in the North West, became operational in June.