The Welsh Government has launched an 18-month study into road-based solutions for congestion on the M4 around Newport, with options assessed by Arup ranging from £14m to £1.8bn.
Arup’s review found that around 25% of peak traffic would need to be removed from the corridor to avoid congestion, based on projected conditions in 2042. None of the individual measures it assessed reaches that level.
Deputy Minister for Transport Mark Hooper said the government was “taking action to reduce peak-hour traffic by up to around one in ten vehicles”. A preferred option will be selected after the study.
Options ranked by cost
Arup’s review, carried out between June and August 2026, builds on existing evidence and contains no new traffic modelling. Its cost and traffic figures are therefore indicative, and the inclusion of an option does not confirm that it is feasible.
- £14m: closing junction 27 and the east-facing slip roads at junctions 25 and 26. Arup estimates a 3% reduction in M4 traffic, with delivery in two years.
- £30m: additional rail services on the South Wales Main Line, producing a 1% reduction over five years.
- £49m: signing and HGV rest facilities intended to direct traffic towards the A465. The estimated reduction is 1%, with delivery in three years.
- £63m: the junction closures combined with the A465 measures. Arup describes this as its fastest and cheapest combination, with a reduction of up to 4% in less than three years.
- £335m: a Newport–Cardiff bus rapid transit route, estimated to cut M4 traffic by 2% over five years.
- £800m: six new stations between Cardiff and Severn Tunnel Junction, with a 6% reduction over six years.
- About £1.2bn: all three public transport measures together, estimated to reduce M4 traffic by 7% over six years. Adding the £63m package would bring the total to about £1.23bn and up to an 11% reduction.
- £1.7bn: widening the Brynglas Tunnels. This would add capacity rather than remove traffic and is expected to take 12 years.
- £1.7bn: targeted improvements to the A48 Southern Distributor Road, estimated to reduce M4 traffic by 5% over eight years.
- £1.8bn: dualling both the A48 Southern Distributor Road and the A4810 Steelworks Access Road, with a 6% reduction over eight years.
Three-billion-pound combination
The strongest combination tested by Arup would cost about £3bn and bring together the A48 and A4810 works, junction closures, the A465 measures and all the public transport options. It could reduce M4 traffic by up to 17% over eight years, still below the estimated 25% requirement unless additional road capacity is provided.
The Brynglas proposal would use the hard shoulder to create a four-lane motorway between junctions 29 and 26. A new westbound three-lane tunnel would be built, while the existing tunnels would carry three eastbound lanes. The speed limit would remain at 50mph.
Arup rates the tunnel scheme as presenting a “high engineering/geotechnical risk” and estimates that it would cause around seven years of significant disruption on the local road network.
Impact of junction closures
Arup says the close spacing of junctions 25 to 27 creates merging and weaving as local traffic joins and leaves the motorway. Closing the junctions would keep some local trips off the Brynglas section but would displace traffic onto roads around Malpas, High Cross and Ridgeway.
The review recommends that the closures could first be trialled using an experimental traffic regulation order. It also warns that shorter journey times may encourage additional traffic, reducing the initial benefit.
A new motorway and a Magor–Castleton tunnel were excluded from the review because of their “affordability, environmental and planning challenges”. Tolls and road-user charging were also outside its scope because the Welsh Government is not currently pursuing them.
Five-year programme alongside study
A separate five-year programme will run alongside the study. It includes motorway renewal and maintenance, a plan to improve operation and management of the route, targeted improvements around Newport, greater use of the A465 for freight, and support for Cardiff Parkway and Magor and Undy stations.
The study’s own cost has not been disclosed. The Welsh Government is expected to select a preferred solution once the 18-month process is complete.