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Ramp Metering

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Ramp Metering
M42 J2 Hopwood Park.jpg
Ramp Metering at M42 J2
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Ramp Metering are part-time traffic lights installed on slip roads in order to control the flow of traffic joining a major road, most usually a motorway. These traffic lights simply hold traffic on the slip road in an attempt to mitigate the congestion effects of traffic joining the major road by allowing traffic to flow along the major road and help minimise flow collision.

Initial Trials

The first installation of ramp metering in the United Kingdom was installed in May 1986 on the M6 at junction 10 between Wolverhampton and Walsall, initially only on the southbound side but it was extended to include the northbound entry soon afterwards[1]. The trial cost £200,000 and used gantries to improve visibility.

The Department of Transport soon called the trial "really impressive", commenting that it had eliminated the 4.5 mile tailback that usually builds up. The system was extended further south along M6 at Junction 9 near Wednesbury, Junction 7 with the A34 between Walsall and Birmingham, and Junction 5 to the east of Birmingham in 1988. Whilst the initial trial installations showed substantial benefits, the system was not extended further along the road network at that time.

Further trial sites

Further trials were commissioned in 1998, with the original sites gaining upgraded equipment and further trial sites on the M3 (at junctions 3, 11 and 12) and M27 (at junctions 3, 5, 7, 10 and 11) were selected, with the new equipment being installed in 2000. These used verge-mounted signals with blue diamond backgrounds. These further trials were partially successful - the M6 sites showed improvements, whilst the results from the M3 and M27 were more mixed. Sufficient success was shown for a further trial roll-out in 2007 to 30 sites on the motorway network:

The 2008 conclusions of this final trial were that Ramp Metering had a positive impact on traffic flows on the mainline of the motorway, and that the delays to traffic on slip roads were outweighed by this improvement. Perhaps not surprisingly, the trial concluded that the more traffic was delayed on the slip road (and hence the longer that the junction was effectively closed) the more the traffic flow in the mainline improved.

References

A-roads

The junction at Wooden Bridge on the A3 in Guildford and the Teesside Park junction on the A66 in Stockton heading eastbound are thought to be the only true examples of part-time ramp metering outside of the motorway network.

Ireland

Given that Ireland's motorways are generally much less congested, the country only has three examples of motorway ramp metering, and they all operate differently to the UK examples.

Two of the examples are at Santry Rotary, where the signals are set to the Irish 'flashing amber' 24/7, and only activate as signals when an incident has upset the traffic flow leaving the Dublin Tunnel. The presence of the ramp metering allows traffic leaving the tunnel to be given priority, reducing the likelihood of congestion (and therefore collisions) in the tunnel.

The third example is in the same area. It is at the M1/M50 Interchange, and is only switched on when there is stationary traffic on the approach to the tunnel. This helps staff on the ground manage the traffic by separating port-bound HGVs from city-bound cars.

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Ramp Metering
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