Planners are being urged to rethink building an M4 relief road in south Wales after research showed travel jams cost the nation £40m a year.
It was announced last summer that the £1bn relief road would not go ahead.
But a Cardiff Business School academic told BBC Wales' Week In Week Out that the road around Newport must be part of a route linking Ireland to Europe.
The assembly government says it simply cannot afford the project, but it is working on alternatives.
The motorway is used by 38m people every year, many more than for whom it was originally designed.
Week In Week Out investigates the impact of motorway congestion and roadworks on businesses and motorists, and what is being done to tackle the problems.
Dr Anthony Beresford, from Cardiff University's Business School, said the M4 in Wales was worth about £2bn a year to the country's economy, and put the cost of congestion at about £40m.
"The M4 is absolutely vital to the economy. To Wales as a whole and in particular to south Wales. We simply couldn't do without it," he said.
In July, the Welsh Assembly Government decided not to go ahead with plans for an M4 relief road to the south of Newport after the cost soared to £1bn.
But Dr Beresford said that decision should be overturned at some stage.
"I think in the longer term, an M4 relief road needs to be constructed. Putting this new route around Newport has to be part of the improved trunk route from Ireland to England - and onto the continent," he said.
The academic told BBC Radio Wales the cost of the M4 congestion could "quickly get into the hundreds of millions if you look at the cumulative cost".
He said: "I think it's always worth investing in new roads in the same way that you invest in new computer equipment and new staff and so on, so we can't write off new building forever.
"Obviously the economy we've got at the moment is somewhat stifled but we see release of pressure in certain areas, so I think it's a matter of time before we do see major investment projects coming back on to the agenda."
While the relief road has been scrapped, the Welsh Assembly Government is committed to trying to improve the M4.
Widening work north of Cardiff is nearly completed, safety barrier work is ongoing and variable speed cameras should be working in time for next year's Ryder Cup at Newport's Celtic Manor resort.
Then there are plans to link up a dual carriageway which runs through the Corus site at Llanwern, with the Southern Distributor Road, to provide an alternative route around the south of Newport.
All the improvements will mean motorists face up to four years of road works.
Average speed cameras are currently in place to ensure motorists stick to 50 mph between junctions 24 at Coldra, and 28 at Tredegar Park, as the road is improved.
Variable speed limits
A petition, currently signed by over 50 people, has been submitted to the assembly calling for the limits to be scrapped.
The assembly government has previously said these will be replaced by variable speed limits in time for the Ryder Cup in Newport next October.
The hi-tech system - which is already used on some English motorways but will be a first in Wales - changes speed limits in response to road conditions.
It uses traffic sensors calculate speed limits, which are displayed on overhead signs.
Deputy First Minister Ieuan Wyn Jones said: "I understand how frustrating it must be for people, during the work they are being delayed but I think at the end of the day we will have a much better network.
"We simply couldn't afford to build the new road [M4 relief road] and actually we can do a lot of the work a lot quicker, and actually provide people with the alternatives that they need."
Week In Week Out: Motorway Madness is on BBC One Wales on Wednesday, 25 November at 2245 GMT.