Newport M4 traffic eased 'for almost a third the cost'
Congestion on the M4 around Newport could be eased for almost a third of the cost of the Welsh government's motorway proposals, a think tank says.
Consultation has been taking place on plans for a £1bn relief road to be financed under new borrowing powers.
The Institute for Welsh Affairs (IWA) says upgrading the A48 and a road through the city's former steelworks is an "affordable" £380m alternative.
Ministers have said other options do not offer "a long-term solution".
Meanwhile, two eco-charities have called the relief road consultation which ends this month "unlawful" as it does not offer more options to consider.
Gwent Wildlife Trust and Friends of the Earth Cymru say they will consider a legal challenge if other proposals - in particular the one outlined by the IWA - are not included.
The IWA is billing its proposals, called the Blue Route, as a "cost effective" solution to the problems affecting south Wales' main traffic network.
Ministers have said that cutting motorway congestion in the Newport area is essential for the Welsh economy.
In April this year, Chancellor George Osborne confirmed his backing for an M4 relief road, describing it as one of the most important road schemes in the UK.
UK ministers announced in November that the Welsh government could borrow the estimated £1bn needed to fund the project.
But Prof Stuart Cole, who wrote the report for the IWA and the Chartered Institute of Logistics and Transport, says the Welsh government is over-estimating likely traffic demands.
He said ministers are also not taking account of public transport advances such as the electrification of the Wales-London rail line and a possible metro-style train system for Cardiff and surrounding areas.
"The issue is whether the Welsh government's present motorway option provides an unnecessary increase in capacity and in consequence unnecessary expenditure," said Prof Cole.
"The Blue Route would deliver what is needed at a much lower cost and with significantly less impact on the environment."
The report quotes the Welsh government's consultation paper as seeing a need for 20% more traffic capacity by 2035 on the motorway around Newport.
Prof Cole said: "Electrification of the South Wales mainline alone would reduce M4 peak traffic flows by up to 15%.
"The Blue Route would solve the congestion issue on the M4 as it arises. Moreover, since it could be built sooner than the motorway it could ease congestion earlier.
"Combined with the metro and rail electrification it would provide more than adequate relief to congestion over the period to 2035."
IWA director Lee Waters said: "The Welsh government has repeatedly said that sustainable development is 'the central organising principle' of its economic policy.
"The decision it makes on a new M4 will be the test of that.
"If the Welsh government means what it says about sustainable development it must take the proposal seriously."
After the challenge lodged by Gwent Wildlife Trust and Friends of the Earth Cymru, a Welsh government spokesperson said: "We are currently consulting on options to improve the M4 around Newport. The closing date for responses is 16 December.
"Other options had been previously considered but were not taken forward in this consultation as they would produce very little relief to motorway congestion and not provide a long-term solution."