Both the Queensferry Crossing and Forth Road Bridge will be closed overnight to test the implementation of an emergency diversion.
From 10pm on Saturday until 8am on Sunday all traffic will be diverted via the Kincardine Bridge.
The test, by Transport Scotland’s operating company BEAR Scotland, will add an estimated 36 miles and 40 minutes to affected journeys.
The west footpath on the Forth Road Bridge will remain open to cyclists and pedestrians.
In February, the Queensferry Crossing closed for the first time since it opened in 2017 after ice and snow fell from cables onto vehicles below.
On Friday, the £1.35bn bridge closed again for more than four hours due to falling ice after “thundersnow” hit parts of Scotland.
Chris Tracey, BEAR Scotland’s south east unit bridges manager, said: “The purpose of this closure is to test how long it will take to implement an emergency diversion via the Forth Road Bridge in the event that the Queensferry Crossing has to be closed to traffic.
“This is a complex process involving removing roadside barriers and laying a large number of cones and signs, so we need to carry out a trial in order to measure precisely how long it will take and refine our procedures. This will allow us to implement a diversion as quickly and efficiently as possible when required.
“It has always been our intention to use the Forth Road Bridge as an emergency diversion route once its full capacity is available. We’ve been working with Police Scotland and our traffic management contractors to plan this test for some time.”
Ongoing work to replace the main expansion joints on the Forth Road Bridge means that it is not yet available, however work on the southbound carriageway is expected to be completed within the next few weeks.
Mr Tracey added: “On this occasion we’ll therefore just be testing how long it takes to open up the diversion and close it again, without directing any traffic over the Forth Road Bridge. A further trial with traffic diverted over the Forth Road Bridge will be carried out later this winter.
“I’d like to thank bridge users in advance for their patience and understanding. We’ve scheduled this closure overnight in order to minimise disruption, however our advice if you do need to travel at this time is to plan ahead and allow extra time for your journey.”