The existing bridge is due to be reserved for public transport and cyclists
Campaigners have told a committee of MSPs that a new Forth road bridge would not be a "justifiable or credible" use of £2bn of public money.
Holyrood's Transport, Infrastructure and Climate Change Committee also heard concerns about congestion on the approach road to the new bridge.
The current road bridge is due to be used exclusively for public transport and cyclists if the new one is built.
Construction is due to begin next year should the project be approved.
The new bridge is scheduled to be completed five years later at a total cost of between £1.7bn and £2.3bn.
But when he appeared before the committee on Tuesday afternoon, Lawrence Marshall, who chairs campaign group Forthright Alliance, hit out at the proposed new bridge.
Mr Marshall, a former chairman of the Forth Estuary Transport Authority, said: "No amount of increase in public transport use of the existing crossing will ever justify the £2bn expenditure and the denuding of the existing bridge of over 90% of its traffic."
"It's not a credible use and not a viable or even justifiable use of public expenditure - to achieve the aim of a clogged-up bridge that's new and a severely unclogged bridge that's old.
"It's so unclogged it's empty, but when you get off the bridge on either side, you're actually back to being clogged up.
"It's like an arterial bypass but none of the other stuff in your body has been sorted."
MSPs had earlier been told by Steve Walker, operations director with Stagecoach, that a "wider problem" for bus firms would be getting to the bridge from both sides of the Firth.
"We have concerns with the current road layout on either side of the bridge," he said.
George Mair, of the Confederation of Passenger Transport UK, added: "It's great that we get through the bridge quickly but we need to think of the issues at either end of the bridge and how we address that."
The future of the current bridge has been thrown into doubt over deteriorating cables, although work to address corrosion is under way.
The Forth Crossing Bill Committee is due to meet on Wednesday, when it will hear evidence on Stage One of the bill from councils and business leaders.
Speaking ahead of the meeting, Liz Cameron, chief executive of Scottish Chambers of Commerce, said the new crossing was a "necessity not an option".
She added: "The economic cost of not acting, which we estimate to be in excess of £1.5bn, simply cannot be contemplated.
"Scotland needs a new Forth Crossing fit for the 21st century and we need to have this project in motion sooner rather than later."