The existing road bridge is set to become a "public transport corridor" once a new crossing is opened to traffic in 2016.
Critics today said the low levels of predicted traffic meant the bridge was set to become the "world's most expensive bus lane", costing the equivalent of 114 per vehicle.
According to figures produced by Transport Scotland for a series of public exhibitions, the new bridge will carry 92,000 cars every day in 2017, with more motorists crossing the Forth due to improved journey times.
Since the abolition of tolls in 2008 all funding for the existing bridge has come from the Scottish Government, with a 112m capital plan for improvements including cable dehumidification and tower strengthening work committed to over the next 15 years.
Lawrence Marshall, chairman of the ForthRight Alliance, which is campaigning against the new bridge, said: "Given the amount of money being spent on retaining the original crossing, it would be the most expensive bus lane in the world by a long, long way."
He added that growing numbers of cars on the new crossing would lead to pressure for the older bridge to be opened up to all forms of traffic.
"It's inconceivable that you could have a position where people are sitting in traffic jams on the new bridge looking across at the old bridge going largely unused."
Transport chiefs managed to shave around 2 billion off the original 4bn price tag for the new crossing by dropping plans for a bus or tram lane and reducing approach roads from three lanes to two in 2008.
Margaret Smith, Liberal Democrat MSP for Edinburgh West, said the scheme as it stands had been "cobbled together" as a "cost-cutting exercise".
She added: "The level of traffic on the old bridge means there's going to be an absolute clamour for both these bridges to be opened to general traffic.
"That would lead to an increase in the road traffic coming over the bridge and into Edinburgh."
A spokeswoman for Transport Scotland said: "Ministers took the decision to retain the existing Forth Road Bridge for public transport and this has allowed the development of an operationally flexible, narrower, replacement crossing of high quality at a lower cost."