From Roader's Digest: The SABRE Wiki
|Birmingham's A38(M) features Tidal Flow|
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Tidal flow refers to a road where a lane or lanes can sometimes carry traffic in one direction and at other times in the opposite direction, to help with traffic flow. Traffic signals usually indicate when a lane is open or closed.
Tidal Flow is also sometimes installed in tunnels or bridges, to allow traffic to be diverted around an incident.
Tidal Flow tends to be used on roads where the flow of traffic is heavy in a certain direction and light in the other at some times, and then reverses at other times (e.g. commuter routes). It allows the road space to be used the most efficiently.
Overhead signals will show either a white or green downwards arrow when a lane is open and a red cross when a lane is closed. Some can also show a white diagonal arrow to show a lane is about to close, pointing in the direction that a driver should move towards. Some routes , such as the A470 in Cardiff, also have coloured cats eyes which glow red to show a line that cannot be crossed, and white between traffic lanes going in the same direction.
Roads with tidal flow
|Birmingham||The A38(M) Aston Expressway in Birmingham is the most famous example, being a single carriageway motorway without physical barrier down the centre of the road for much of its length: instead overhead signals show which lanes are open and closed, with one lane always closed to separate traffic heading in opposite directions.|
|Plymouth||Covers both the Tamar Bridge and Saltash Tunnel|
Former Tidal Flow Systems
|London Road, Manchester|
|Upper Brook Street, Manchester||Removed in 1989-90, although the overhead gantries and differently coloured centre lane lasted for a couple of years following this, being finally removed entirely circa 1993.|
|Blackwall Tunnels, London||The Blackwall Tunnel only ran tidal flow in the AM peak period. Tidal flow was implemented at the discretion of the police. It was only usually implemented between 0630 and 0900 hrs, and only ran for as long as it took to reduce the queuing on the south side - run it for too long and the southbound traffic would begin to back up. During the morning peak, tidal flow could be implemnted up to three times, for as little as 20 minutes at a time.
It was not possible to run tidal flow in the PM peak because this would involve running contraflow in the northbound tunnel. This is the original 1897 tunnel and has some particularly sharp bends in it - not suitable for two-way traffic. Tidal flow ceased to operate in April 2007 because of concerns about the number of collisions within the tunnel.