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Tidal Flow

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Tidal Flow
A38(M) For Birmingham - Near The End, Birmingham Skyline - Geograph - 1291182.jpg
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.


The A15 with tidal flow signals above each lane

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.

While tidal flow systems have generally fallen out of favour in the UK, a Highways England report gave serious consideration to installing one on the A3024 at Bitterne railway station in 2018. The idea wasn't pursued, but it was still remarkable that it was ever considered.

Roads with tidal flow

Image Road Location Notes
A38(M) For Birmingham - Slip Road For Aston, Perry Barr and Birchfield - Geograph - 1291173.jpg Chopsticks icon.png A38(M) 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.
Pier Terrace, Lowestoft - Geograph - 2961817.jpg A12 Lowestoft
A15 Lincoln, Canwick Road Tidal Flow 1.JPG - Coppermine - 12560.JPG A1434 Lincoln Previously A15 until 19th December 2020 when the A15 Lincoln Eastern Bypass was completed and the A1434 was extended along its former route.
Tamar Bridge facing East - Geograph - 151050.jpg A38 Plymouth Covers both the Tamar Bridge and Saltash Tunnel
A470 Cardiff
Lakeside North Harbour (unclassified) Portsmouth A rare example from a private road, but one that is used by many businesses and a Park and Ride site. It used to have a strange three-into-one lane drop, but in the early 2000s the road layout was changed and the centre lane was marked 'peak time only' in both directions. Initially this was enforced with cones, but by 2007 they had been forgotten about.
Alton Towers (unclassified) Alton Towers The road that connects the car parks to the road network is a very odd Tidal Flow layout which can be confusing as the lane markings show clearly that it should be two lanes to get out while 1 lane coming in, but signage shows the opposite.
Leopardstown Racecourse (unclassified) Carrickmines The link to the M50 is S3, with the racecourse managing its flow.

Former Tidal Flow Systems

Image Road Location Notes
Portsmouth tidal flow.jpg A3 Commercial Road, Portsmouth
London Road, Manchester - Geograph - 696295.jpg A6 London Road, Manchester
Entering Manchester on Upper Brook... (C) Ben Brooksbank - Geograph - 3392290.jpg A34 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.
A102 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.

The Queens Road Tidal Flow system on the A61 in Sheffield. OK, it's a slightly dark photograph, but the lighting effects on it just seemed great in my humble opinion. The sunset was great that day. - Coppermine - 4607.jpg

A61 Sheffield Removed in July 2019 following concerns on the system being harder to maintain. The Tidal Flow has been replaced with two permanent inbound and outbound lanes

Tidal Flow
Related Pictures
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A15 Lincoln, Canwick Road Tidal Flow - Coppermine - 12571.JPGA15 Lincoln, Canwick Road Tidal Flow - Coppermine - 12570.JPGA15 Lincoln, Canwick Road Tidal Flow - Coppermine - 12566.JPGEntering Manchester on Upper Brook... (C) Ben Brooksbank - Geograph - 3392290.jpgA38(M) For Birmingham - Near The End, Birmingham Skyline - Geograph - 1291182.jpg
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