Congestion fees may be introduced either side of the Severn crossings after bridge tolls were scrapped.
The number of vehicles using the two bridges after the tolls ended in 2018 has increased and probably made congestion worse, the West of England Combined Authority (WECA) said.
The authority is considering "charging measures and controls" on both sides of the bridge to deal with traffic levels.
No timescales have been given for when charges could be introduced.
The ideas are being considered as part of its latest joint local transport plan, which sets out infrastructure plans up to 2036.
But the suggestion has been denied by the UK government, who said there were "no plans" to reintroduce tolls.
In 2017, former prime minister Theresa May pledged to scrap the bridge tolls when the structures returned to public ownership.
Prior to this, cars were charged £5.60 to cross from England in to Wales.
The tolls were abolished on 17 December 2018.
The transport plan also considers new bus routes, park and rides, and junction improvements to relieve traffic pressure.
A WECA spokesman said: "The removal of the tolls provides many opportunities to boost the economies of the West of England and South Wales.
"We are working with the Department for Transport, Highways England, Wales Office and Welsh Government - as well as our constituent councils - to ensure we make the most of these opportunities, while mitigating any potentially negative impacts.
"This includes considering measures to reduce traffic flow and cut congestion."
But the report also adds improvements to public transport may not be sufficient enough to counteract the rise in traffic.
"The implications of removal of the Severn Crossing Toll are a key unknown," it adds.
A Welsh Government spokesperson said: "We have had no discussions about the reintroduction of tolls with Highways England but are continuing to work with it to understand the impact of removing the tolls on congestion in south east Wales.
"The South East Wales Transport Commission is continuing its work to examine the longer-term solutions to congestion along the M4 Newport corridor."