An extra £45m has been added to the price tag for the Norwich Western Link, pushing the cost up from £153m to just under £200m.
Norfolk County Council is on the brink of awarding a contract to build the road, but has revealed the estimated cost has gone up to £198m - due to inflation, market forces and an extra £22m for environmental mitigation.
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But critics say the price, both environmentally and financially - at £50.8m a mile - is not worth paying. They say the road, which would connect the Norwich Northern Distributor Road to the A47 to the west of the city, should be scrapped.
The new details were revealed in council papers which will inform a decision by the council's Conservative-controlled cabinet to submit the outline business case for the 3.9 mile road, which would go from the A1067 and link to the A47 at a new junction at Wood Lane near Honingham, with a 720-metre-long viaduct over the River Wensum.
That cabinet meeting, on June 7, will also agree to award a contract to a company to build the road, although the council says, for confidentiality reasons, the identity of the contractor, will not be revealed yet.
Seven companies expressed an interest, which was whittled down to three. Council officers say a "robust" procurement process drilled down into their tender offers, which was part of the reason for a previous decision being delayed.
Officers say that, because the contractor will also design the road, that reduces the risks to the council of the cost increasing again. The council had previously seen the price of the NDR rise because extra costs were added during the course of the contract.
They say the road scores highly on value for money criteria used by the Department for Transport, while an extra £22m will be spent on further environmental mitigation, including more green bridges and tunnels for wildlife.
That comes following criticism from Norfolk Wildlife Trust, the Norfolk Rivers Trust and the Norfolk branch of the Campaign To Protect Rural England over the road's impact on wildlife. The council's claim that mitigation would mean a net gain for biodiversity has been questioned.
And independent bat experts, Wild Wings Ecology, say the road would wipe out what they say is the largest barbastelle bat colony in the UK.
The cabinet papers which will go before councillors acknowledge those concerns, but the council says it is continuing its own bat surveys.
The council also claims the road will lead to a reduction in more than 450,000 equivalent tonnes of carbon dioxide over 60 years - based on projections for future uptake of electric vehicles.
Martin Wilby, the council's cabinet member for highways, transport and infrastructure, said: "This is not just about building a road, we are taking into account the environment in the area. We take that very seriously.
"We want to show we can build a road and improve the biodiversity at the same time and we are confident that we can.
"This will bring investment into Norfolk, with 85pc of that coming from the government, which will help with the economic recovery following Covid-19."
He said it would also ease congestion in areas to the west of Norwich, including Weston Longville.
But Emma Corlett, deputy leader of the opposition Labour group, said: "The Western Link remains environmentally and financially reckless. The time to end this is now, before we expose the council tax payers to any further increases.
"I do not think any amount of money would be enough to mitigate against this environmental catastrophe."
She said the claim it would lead to a carbon reduction was "disingenuous", as that does not include carbon emissions resulting from construction.
Owners of woodland on the route are also opposing the scheme, while the Stop The Wensum Link campaign group is campaigning to prevent it.
Supporters of the road include the Norfolk and Norwich University Hospital, Norfolk Chamber of Commerce, Norwich Airport, Norfolk Fire and Rescue, the New Anglia Local Enterprise Partnership, Road Haulage Association, First buses, most of the county's MPs and a number of district councils. However, Konectbus, which had backed the road, has now switched to a neutral stance.
The Labour and Green groups at County Hall oppose the road, while Norwich City Council withdrew its 'in principle" support for the road in January.
Jamie Osborn, Green county councillor, said the scheme would not cut carbon emissions. He said: "They know the figures do not stack up in terms of the money and carbon or the environmental impacts. And they are trying to push it through without scrutiny."
The Department for Transport gave conditional support for the scheme last summer and the green light to proceed to the next stage of the national process - the Large Local Majors scheme.
But permission and funding has yet to be secured, which is why the business case to the government is so key to the council's hopes.
Government backing would contribute about 85pc of the total price of the project, with the council likely to borrow millions more to cover the rest of the cost.
If agreed by cabinet, and then by a meeting of the full council directly afterwards, a pre-planning public consultation will start in the autumn, while a planning application follow early next year.
If funding is secured and the road granted permission, work could start 2023 and it would open to traffic in 2025.
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