Widening the M25 to ease congestion in south-west London has been rejected in a government report.
Efforts to improve journeys on the south-west quadrant of the orbital road should instead focus on encouraging more people to use public transport or "find alternatives to travel", according to the Department for Transport and Highways England study.
But it accepted road enhancements are "likely to be needed" because of the volume of traffic.
This part of the M25 has consistently been the busiest road in the UK since it opened in the late 1980s.
Severe congestion is a regular occurrence with a "12-hour peak period" effectively lasting from 6am to 6pm.
The busiest portion, between Junctions 14 and 15 near Heathrow Airport, is used by an average of 220,000 vehicles each day.
Bridget Fox, of the Campaign for Better Transport, claimed the government had recognised that "we cannot simply build our way out of congestion".
She said: "The M25 study is right to focus on improving capacity on the existing network through moving to more sustainable modes.
"That makes sense for the environment and the taxpayer too.
"Those measures should be implemented in full before any new roads are planned.
"New roads create more traffic, and it is wrong to sacrifice precious countryside for more tarmac when positive alternatives are available."
Steve Gooding, director of the RAC Foundation, described the motorway as a "victim of its own success".
He went on: "There's only so much that can be done on the M25 itself to ease congestion.
"In places it is already 12 lanes wide.
"While capacity can still be improved at the margins - through the rethinking of some of the junctions for example - the government is right to look beyond the motorway and consider with councils whether local roads and rail links are up to the job.
"After all, there isn't a single trip that begins and ends solely on the M25, and many drivers would like to avoid it completely."
The report is part of the DfT's road investment strategy to improve motorways and major roads.
When the M25 was being planned there was talk of it being only two lanes wide, but it opened with three lanes in each direction, with various widening schemes introduced as the years went on.