The Forth Road Bridge is running close to capacity and could fail to cope with traffic levels within six years, according to an independent report.
Consultants said another crossing would alleviate congestion
The study, commissioned by councils in south east Scotland, said increased tolls and public transport improvements would only have a short-term effect.
Instead, the consultants supported the idea of building a new crossing, which could cost more than £700m.
A study is already under way into how and where it could be built.
The findings came ahead of another weekend of expected disruption for travellers crossing the bridge due to maintenance work.
From 1900 BST on Friday a contraflow system will be in operation which is likely to lead to long delays.
The study predicts delays on the bridge are going to get worse and by 2011 the bridge will not be able to cope.
It said that even introducing a £4 toll for driver-only cars at peak times would not be enough to halt the growth in traffic.
The current programme of maintenance on the 41-year-old bridge has led to a series of closures and restrictions.
The Forth Estuary Transport Authority (Feta) said the work was ahead of schedule and was now being carried out over fewer weekends.
However, there is likely to be serious disruption this weekend and similar restrictions will be in place a fortnight later.
Barry Colford, deputy general manager of Feta, advised drivers who must make journeys this weekend to consider alternative routes or to use public transport.
He said: "It is best to avoid travelling via the Forth Road Bridge. If people take this advice, the delays shouldn't be too horrendous.
"We have liaised with the Scottish Executive and its trunk road contractors, so there will not be any conflicting roadworks in the Kincardine Bridge area while this essential maintenance work is under way at the Forth Road Bridge."
Meanwhile, more tests have been ordered on the bridge after inspections of its suspension cables proved inconclusive.
The thousands of wires which make up the cables were opened up at 10 locations, and a small number were found to be broken or corroded.
However, there is not enough information to be able to predict how much longer the cables will last.
Audio monitoring equipment which is already in use on some American bridges is to be installed next year to give engineers more information.