Traffic restrictions, including lane and slip road closures, will be in place between Boxing Dayand spring 2012 so the third phase of a £30million upgrade of the White Cart viaduct south of the airport can take place.

The road is used by more than 90,000 vehicles a day and the work is expected to cause significant disruption for thousands of holidaymakers and airport workers for whom the M8 is the only major access route.

The White Cart viaduct, built in 1968, had a 40mph speed limit imposed in 2007 after structural analysis highlighted weaknesses in the structure which led to the lanes being narrowed as a precautionary measure.

This phase of work costing £11.8m, was due to take place a year ago but work was delayed due to the prolonged snowfall the snow and sub-zero tem- peratures last winter.

Average speed cameras will be installed on December 9 to ensure motorists stick to a new lower limit of 40mph. They will be removed in spring when the speed limit reverts to 60mph. A fourth and final phase of the programme – including painting external steelwork, concrete repairs and below-deck maintenance – will follow and is expected to be completed by 2016.

Transport Scotland, the Government agency overseeing the work, said a "worst case" scenario would entail delays of up to 60 minutes at peak periods. A spokesman said work on the carriageway had been planned to start in January to coincide with the least busy period for the airport.

"We have specifically chosen to start the daytime element of these works on December 26, because our data shows this is when there is the least number of heavy goods vehicles on this route," the spokesman said

"Thereafter, there is a window of opportunity to complete the works which minimises the impact on other stakeholders such as airport travellers."

Since July work has been carried out underneath the bridge. Overnight lane closures have been in place since the beginning of this week and will continue until the start of the Christmas holidays.

Traffic restrictions put in place between December 26 and January 9 will see three carriageways in each direction reduced to two. Similar closures are expected to follow.

The approach ramps at junctions 27, 28 and 29 will be closed on an intermittent basis but the exits at all three junctions are expected to remain open.

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