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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.
The first installation of ramp metering in the United Kingdom was installed in 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. The initial trial was considered to be successful, and 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.
Futher 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 5, 7, 10 and 11) were selected, with the new equipment being installed in 2000. 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.