Scrapping tolls on two major road bridges will generate extra traffic and bring longer delays, a report said today.
The warning came from Executive-commissioned consultants who recommended the plan be scrapped.
But this was turned down flat by finance secretary John Swinney.
"The government is clear that it would be an injustice to the communities of Fife, Tayside and the Lothians to keep tolls when elsewhere in Scotland they have removed," he said.
The report was published as public exhibitions showing the possible replacements for the Forth Road Bridge opened today.
The report also coincided with the latest annual transport statistics, showing the number of vehicles on Scotland's roads is the highest ever recorded at 2.29 million.
This increase came despite a 3% fall in the number of new vehicles registered in Scotland in 2006.
The transport statistics also show the volume of traffic on Scotland's roads, at 44 billion "vehicle kilometres", was greater in 2006 than in any previous 3% up on 2005 and 16% more than in 1996.
But the figures also show that the number of road casualties, 17,165, was the lowest figure since 1952 and the number seriously injured in road accidents, at 2,611, was the lowest since these records began in 1950.
The transport statistics also show rail passenger journeys reached 77.3 million, their highest level for more than 40 years.
And the number of passengers using Scotland's air terminals in 2006 was also the highest on record, at 24.4 million.
The recommendation not to scrap Forth and Tay tolls came from consultants commissioned by the previous Labour-Liberal Democrat Executive to study the impact of removing tolls.
They predicted "significant" extra travel movements, longer travelling time with associated costs, and an environmental impact.
But over the long-term scrapping tolls could keep 1,000 more people in Fife in jobs, the report said.
Plans to scrap the tolls were approved by MSPs on May 31, when ministers pledged they would be removed as soon as was practicable.
The exhibitions setting out the proposals for a replacement for the Forth bridge - a new bridge or a tunnel to the west of the existing structure - opened today in Edinburgh and at North Queensferry.
Mr Swinney said the project would be one of the biggest ever seen in Scotland.
"It is vital to Scotland's future, and with such an important decision I am very keen to hear people's views on the proposals.
"But if we are to keep Scotland moving in years to come, and achieve our aim of improving economic growth, it is clear that we need the public to help us on more than just the decision on the type of crossing."
Mr Swinney said today's reports showed growing road traffic levels.
"We must give people the choice of excellent public transport," he said.
"We will build new rail lines, invest in strategic road links and improve the bus network.
"We also need more people across the country to make the choice to leave the car at home.
"We must encourage change in the mode of transport used by the public to protect our environment and ensure our economy can develop without being constricted by an increasingly congested roads network."
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