The Forth Road Bridge will open to cars on Saturday as emergency diversion plans are trialled for the first time.
The Queensferry Crossing will close and M90 traffic will divert via the old road bridge throughout the day.
The move follows the closure of the £1.3 billion crossing due to ice for the third time in a year last week.
Motorists faced a frustrating 40-mile diversion via the Kincardine Bridge during the seven-hour shutdown.
Saturday's Forth Road Bridge trial will be the first to include live traffic.
An earlier trial in December only tested the implementation of traffic management without diverting cars onto the route.
Bridge will only open outwith peak periods
Queensferry Crossing operator Bear Scotland said any real emergency diversion will only be implemented overnight, outside peak periods.
The new bridge will close at 10pm on Friday and traffic will initially be diverted via Kincardine.
This is to allow transport bosses to open up connections between the Forth Road Bridge and the motorway.
The Forth Road Bridge continued to fulfil an important role as a public transport corridor."
Bear Scotland manager Chris Tracey.
The diversion via the Forth Road Bridge should be up and running by 6am on Saturday at the latest.
All traffic will use the old bridge until 10pm on Saturday.
Traffic will then again be directed via Kincardine Bridge while the M90 over the Queensferry Crossing is reopened.
'Important role'
Bear Scotland manager Chris Tracey said: "The Forth Road Bridge continues to fulfil an important role as a public transport corridor.
"However, it is now also available to provide additional resilience as an emergency diversion route for motorway traffic should the Queensferry Crossing have to close for an extended period.
"We carried out an initial test of this process in December and are now ready to trial the full diversion, with motorway traffic running on the Forth Road Bridge.
"It should be noted that emergency diversions over the Forth Road Bridge will normally only be implemented overnight, outside peak periods.
"This is because the process involves laying large numbers of cones and signs, stopping traffic on the M90 and A90, and moving roadside barriers to connect the bridge directly to the motorways north and south of the Forth.
"This takes several hours and would cause significant additional disruption to traffic during busy periods."
He added: "We will also only direct traffic via the Forth Road Bridge if the Queensferry Crossing is forecast to remain closed for longer than the time it takes to set up the diversion."
Speed limit and high wind warnings
Mr Tracey reminded motorists there is a 40mph speed limit on the Forth Road Bridge.
He also warned that high wind restrictions are more likely due to its lack of wind shielding.
In addition, there are no hard shoulders on the Forth Road Bridge.
"If you break down please remain in your vehicle and our on-site recovery team will get to you as soon as possible," Mr Tracey said.
Last week's closure was labelled a "national humiliation" by Conservative MSP Alexander Stewart.
It followed severe overnight weather, including an amber alert for blizzards.
