Sustrans Cymru - an organisation that promotes greener transport - has condemned the potential M4 Toll as "poor value for money".
A spokesman said that a "more cost effective way to reduce congestion would be to invest in shifting these journeys to public transport."
"The Welsh Government acknowledges that 40% of journeys on this stretch of the M4 are local trips of under 20 miles," he added.
More top news
A British amateur sailor died this morning after being hit by a sail while taking part in the Clipper Round The World Yacht Race.
Plans for seven-day GP services are "unrealistic" because of a shortage of doctors, the Government has been warned.
Forty-one people have been injured, four seriously, after a Welsh tourist coach was involved in a pile-up on a Swiss motorway.