Tidal Flow

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.

Basics

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)	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.
	A12	Lowestoft
	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.
	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
	A3	Commercial Road, Portsmouth
	A6	London Road, Manchester
	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.
	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