Wales' former transport minister has said ambitious proposals to widen the M4 at Newport were deemed unnecessary while he was in office.
Ieuan Wyn Jones was responding to Welsh government proposals for a busy stretch of the motorway which include boring two new tunnels at Brynglas.
Other options for the notorious traffic bottleneck include a new dual carriageway to the south of Newport.
It follows a decision in 2009 to scrap plans for an M4 relief road.
The latest proposals suggest creating new tunnels at Brynglas, a pinch-point where the M4 is currently two lanes in both directions.
The new tunnels would allow the motorway to be expanded to four lanes in each direction for nine miles between junctions 24 to 29 over 15 years, at a cost of about £550m.
The proposals as a whole cover improvements between junctions 23 to 29 and have been drawn up by an expert group, including civil engineers Arup.
Technical documents say the disruption caused by road works could prompt opposition. They estimate 200 to 300 homes and businesses would be directly affected by widening and some would have to be demolished.
Plaid Cymru leader Mr Jones was the transport minister in the previous government who dropped plans to build the M4 relief road around Newport.
He said the only way the relief road could be funded was by tolling the motorway.
"That was never really at the end of the day a realistic financial proposition," he said.
The idea of widening the road and building new tunnels at Brynglas was looked at while he was a minister between 2007 and 2011.
"The advice I had was the works we were currently doing would be sufficient to deal with the potential increase in traffic and let's remember of course that traffic has actually gone down during the recession," he said.
"Most people felt that the works that we currently had in hand would be sufficient, but clearly if it was felt that if such an increase in traffic happened in the next 10 years or so then the potential of the new tunnels was being looked at. But no real work was done on it and no firm costings made."
Motorists and local people have been invited to a series of workshops and exhibitions running until 6 June.
Business leaders have warned that traffic jams in the area have been bad for the economy of south Wales.
Last summer traffic was closed in both directions, causing long delays, when a lorry caught fire inside one of the tunnels.
One proposal would see a dual carriageway being built to the south of Newport. Officials said it would be "demand led and built in phases", and that unlike the originally-intended relief road it would not be a motorway.
Work started last month on turning an old access road through the Llanwern steelworks into a dual carriageway linking the south of Newport with the motorway.
There is also an option for junction improvements on the A48 southern distributor road.
Keith Jones, director of the Institution of Civil Engineers in Wales, welcomed the plan and said it would certainly help cut congestion in the area.
"Of course it would because that's what we call part of the critical infrastructure of the motorway around there," he told BBC Wales.
"We saw last year the regrettable two fires that caused such a gridlock in the area. Something needs to be done."
Conservative shadow transport minister Byron Davies said it was "quite some u-turn" following the Labour-led government's decision to scrap the relief road in 2009.
Liberal Democrat AM for South Wales Central Eluned Parrott said: "While additional tunnels at Brynglas would be the preferred answer we have to consider the impact on local residents who already live close to the tunnels."
The Welsh government said the scheme could be financed under the current devolution settlement.
First Minister Carwyn Jones initially said it could only go ahead if his administration was granted the power to borrow money, but officials later said he misunderstood a question during a BBC Wales interview.
Transport Minister Carl Sargeant said easing the flow on the M4 between Magor and Castleton was "a key priority".
"Improving access to our schools, hospitals and workplaces is essential if we are to improve efficiency and productivity and in turn make us more competitive," he said.