More investment in public transport is needed in south east Wales to ease congestion on the M4, the man asked to look at possible solutions has said.
Lord Burns is leading a commission on the issue after the first minister ditched plans for a £1.6bn M4 relief road around Newport last summer.
He said on Thursday that for many commuters the car was currently the only option for travelling to work.
A congestion charge could be needed to address the issue, Lord Burns added.
Giving evidence to the assembly's Economy Committee, Lord Burns said his commission had looked at traffic patterns on the M4 and found there was "a very clear picture" that the congestion is "to a significant degree a symptom of a lack of alternative transport options".
The congestion was "very much a commuter problem", he said, with around two thirds of the morning traffic heading east to west between junctions 24 and 29 consisting of people travelling to Cardiff and Newport.
"For many people there is no alternative or convenient way of getting to and from work other than by car," he said.
"The costs would be much higher and the journey times much longer if they tried to go by some other means."
Lord Burns said his "overwhelming conclusion" was that "the public transport network is heavily under-invested in compared to what is needed".
He also said the flow of traffic on the M4 had increased "quite markedly" since the tolls were scrapped on the Prince of Wales bridge in December 2018.
Asked about congestion charging, Lord Burns said it was important to provide people with feasible alternatives first.
"If you provide people with alternatives then I think road pricing is not only possible but it could well be a necessary part of the package."
Cardiff Council has recently revealed proposals for introducing a congestion charge on drivers travelling into the city.
Lord Burns emphasised that his commission was not being asked to assess how potential solutions would compare with the scrapped relief road scheme.
He also said it would not be looking at a new "motorway-style solution".
The commission hopes to make its recommendations to the Welsh Government before the end of the year.
Lord Burns said it will consider a range of factors including carbon emissions, air quality and value for money.