Drivers heading between Fife and Edinburgh face a diversion across the Forth Road Bridge tonight.
It's taking place between 8pm this evening and 8am tomorrow morning.
A trial of measures is taking place, designed to avoid the need to divert through Kincardine if the Queensferry Crossing has to close again in future.
M90 southbound traffic will be diverted off the motorway at Junction 1B Ferrytoll, before crossing the Ferrytoll Roundabout and joining the Forth Road Bridge via the A9000, which is normally reserved for public transport.
While the diversion is in place, there will be no access to the Ferrytoll roundabout eastbound from the B981 or westbound from Hope Street, Inverkeithing.
Local traffic will have to access the M90 via Junction 1C Admiralty.
Initially, northbound traffic heading towards the Kingdom will be diverted off at M90 Junction 1A Queensferry, then along the A904 Builyeon Road to join the Forth Road Bridge from the Echline Roundabout.
But later in the evening, northbound traffic will be diverted directly onto the Forth Road Bridge, once the emergency crossover has been opened.
Chris Tracey, BEAR Scotland’s Unit Bridges Manager for South East Scotland, said: "Last winter’s two closures of the Queensferry Crossing only lasted for a few hours, so a diversion via the Forth Road Bridge needs to be opened relatively quickly if it is to make a difference.
"In 2021 we carried out two trial diversions via Forth Road Bridge, first using the emergency links on either side of the bridge and then using the slip roads.
"The purpose of these trials was to test precisely how long each of these processes takes.
"This weekend we are trialling opening the diversion in stages, initially via the slip roads and later using the emergency links.
"The aim is to avoid the need for traffic to divert via Kincardine while the diversion is set up.
"Using the slip roads does not provide the same capacity as the emergency links but can be set up more quickly.
"For longer term closures we would still open the emergency links.
"Other measures to speed up the process are also being looked at, including barriers that can be opened automatically.
"We’ll share further updates as this work progresses."