Problems with corrosion were identified 40 years into the lifespan of the bridge
The main suspension cables on the Forth Road Bridge have lost about 10% of their strength, it has been warned.
However, engineers studying corrosion on the bridge said the cables may be deteriorating more slowly than previously feared.
It had been thought weight limits on the crossing could be needed as early as 2014 but latest findings suggest a date of between 2017 and 2021.
A dehumidification system is also being installed to help combat the corrosion.
The first internal inspection in 2004 revealed that the main cables had lost about 8% of their strength and work got under way to fully assess the problem.
Further tests by the Forth Estuary Transport Authority's consultant engineers have indicated a 10% drop, but suggest the deterioration is at the slower end of expectations.
Barry Colford, Feta chief engineer and bridgemaster, said the findings represented the best possible professional assessment using the best available data.
He said: "Until now we only had two points on the graph - 1964 when the bridge was built and the inspection in 2004.
"In the absence of data from the intervening years, it was hard to precisely determine when the corrosion had started and how quickly it was progressing. We now have a third point on the graph, which has allowed our consulting engineers to predict the future rate of deterioration with more confidence.
"The good news is that, although the cables are still losing strength, the worst case scenario of an HGV ban in 2014 now looks unlikely."
In the inspection work, hardwood wedges were driven in to open up the cables, allowing engineers to assess the condition of a representative sample of the 11,618 individual wires which make up each bundle.
The condition of each wire was carefully recorded and samples taken for laboratory testing.
Tensile tests, chemical analyses and corrosion analyses were carried out on selected sample wires.
Feta said the new inspection findings provided an updated benchmark for measuring efforts to halt the corrosion, with a third inspection to be carried out in 2011/12.
Precautionary checks are also being carried out on the bridge's anchorages, where the main cables are attached to post-tensioned concrete-filled tunnels bored into the rock in each shore, to make sure there are no hidden problems.