Abolition of Severn Crossings tolls will worsen congestion on the road network to such an extent that demand for train travel will increase, according to new forecasts.

With the proposed £1.3bn M4 Relief Road around Newport now delayed by two years, motorists and haulage companies face at least five years of even slower journeys on the M4 after tolls are removed at the end of this year.

Motorists seeking an alternative may find that Great Western Railway trains between Cardiff and central Bristol are crowded. Extra services by the imminent new Wales and Borders franchise would have provided relief, but the UK Government ruled them out last year.

Supporters of the M4 Relief Road say the predicted extra congestion strengthens the scheme’s case. Critics say the scheme would drain money which otherwise could improve public transport to reduce road congestion.

Traffic forecasts for the scheme’s public inquiry were originally based on the UK Government’s policy of halving the tolls after the crossings passed into public ownership this week.

Half-price tolls were forecast to increase Severn Crossings traffic by 17% by 2027, on top of a 28% increase from the background increase in road traffic generally, and the UK Government expressed concerns about congestion worsening in Bristol and south Wales.

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Now Bryan Whittaker, the Welsh Government’s traffic expert, has submitted updated forecasts – reflecting toll abolition – to the inquiry, which resumes on January 31.

He says the expected traffic increase is “slightly counteracted” by a shift from private to public transport “as people respond to changes in highway network congestion”.

Although Network Rail is now doubling the number of railway tracks into Bristol Temple Meads from the Newport direction, last year the UK Government told bidders for the Wales and Borders franchise – which starts in October – not to propose any services into Bristol because other upgrade work “may not be completed until 2024”.

Passengers are already dissatisfied with GWR’s two trains per hour between Cardiff and Temple Meads. The next GWR franchise could theoretically include additional trains there, but is not proposed to start until March 2022.

The Freight Transport Association said hauliers welcomed the tolls’ abolition but measures were needed to prevent cars from clogging up the roads. Ian Gallagher, head of policy for Wales and south-west England, said house prices in south-east Wales were already rising as Bristol commuters reacted to the promise of permanent toll abolition.

He said the two-year delay to the relief road was disappointing. “The Welsh Government need to look at what they can do in the interim to encourage people out of their cars and onto public transport.”

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Mr Gallagher, who lives near Bristol, said driving into the city centre in peak periods was a “nightmare”, even with existing traffic levels. “There’s a discussion to be had about the increase in traffic,” he said. “Maybe the Welsh Government or business need to consider flexible working, just to try to smooth that flow of traffic. Does everybody need to travel into work for 8 o’clock or 9 o’clock?”

Stephen Joseph, chief executive of the Campaign for Better Transport, said: “We’re concerned that cutting the tolls will simply increase traffic levels overall. Building more roads to meet projected traffic demand is the definition of unsustainable development. Experience tells us that new roads fill up with new traffic, leaving you back where you started, at great expense.

“We would prefer to see the Government take a broader approach, linking up plans for the Severn Crossings with important policies to cut air pollution, shift more passenger traffic and freight onto rail and reduce transport’s overall carbon emissions.

“We also think the Welsh Government should put real money and priority behind its South Wales Metro proposal, so that it can actually provide a genuine alternative for a lot of the car trips in the area and reduce the need for big road building projects. At present there’s a risk that the M4 project will drain money and priority away from it, leaving public transport users in the area with the leftover crumbs.”

The updated forecasts predict that at M4 Junction 23, immediately west of the crossings, traffic will increase by a further 11% by 2037 as a result of tolls being abolished rather than halved. This includes a 12% additional increase during the morning commuter peak. These figures relate to the “do minimum” scenario, where the relief road is not built.

Mr Whittaker suggests that most of the long-term change in traffic will materialise in the first five years after the tolls are abolished.

The relief road’s official “opening year” is now 2024. Until then, the burden of the increased traffic will fall on existing roads, including the notorious Brynglas Tunnels bottleneck.

By 2022, eastbound journeys in the morning peak between Cardiff Gate and the Second Severn Crossing are forecast to take more than 4% longer than they would if tolls were halved.

Mr Whittaker concludes: “As a direct consequence of the removal of the toll, increased traffic is experienced on the existing M4 north of Newport, bringing about increasing congestion, slower speed of travel and increasing journey times.” Time savings from the proposed new motorway will therefore be greater than if half-price tolls applied. “Accordingly, in traffic terms the removal of the tolls strengthens the case for the proposed scheme.”