Forth Road Bridge: Restrictions until Thursday
Traffic on the Forth Road Bridge will be restricted to a single lane in each direction until at least the middle of the day on Thursday.
Engineers have been inspecting a defect on the bridge, causing lengthy delays for motorists.
The southbound carriageway was closed on Tuesday night as a precaution.
The road network company Amey has revealed there are eight parts of the bridge where a similar problem could be encountered in the future.
There was significant disruption on the M90 and Kincardine Bridge earlier on Wednesday as a result of the defect, with a contra flow currently operating on the northbound carriageway.
One commuter said: "It's been terrible, I came down the Fife route and I was stuck for about two hours."
Traffic Scotland reported congestion on the M90 back to junction 3 (Halbeath), while the A90 was very slow northbound from the M90 beyond Dalmeny.
The A985 and the Kincardine Bridge were also very busy, with a 25 minute delay on the bridge around 08:30.
The problem on the Forth Road Bridge was found during a routine inspection.
Before the bridge was opened by the Queen on 4 September 1964, cars could only get across the Forth on a ferry.
So the Forth Road Bridge, which at the time was the fourth biggest suspension bridge in the world and the longest outside the United States, was welcomed as an engineering marvel.
It transformed the journey between Edinburgh and Fife and from the north of Scotland to the south.
Traffic on the crossing grew steadily and it now carries 24 million vehicles across the Forth every year - far more than it was originally designed to handle.
But its reputation for engineering excellence has been tarnished over the years by traffic congestion and maintenance problems.
The area involved is hard to reach, so a full inspection could not safely be carried out in darkness.
Amey said the footpaths on the Forth Road Bridge will be closed so maintenance vehicles can have access, while abnormal loads are also prevented from crossing the bridge.
High-sided vehicles can now access the bridge after they were initially prevented from doing so due to high winds.
Mark Arndt, from operating company Amey, said: "A key structural component beneath the southbound carriageway has failed so it is important that we carry out all the necessary calculations before relaxing safety restrictions.
"Following the detailed inspection carried out this morning we now have the information required to develop both short and long term solutions. Our team will be working through the night to design and check our plans and we aim to have these confirmed during the day tomorrow.
"We apologise for the disruption that these safety restrictions are causing. Our advice remains to avoid Forth Road Bridge for the time being and divert via Kincardine or the A9 and M9, use public transport or travel outwith peak times.
"Drivers did take heed of the warnings today and we would ask them to keep checking the latest information from Traffic Scotland before they set off on their journeys."
Amey said it would provide further updates when there was new information.
A new £1.4bn Queensferry Crossing over the Firth of Forth is due to open in December next year.
It was ordered by ministers because of corrosion of the main suspension cable on the Forth Road Bridge.