A lane at the A47/A11 Thickthorn roundabout will be shut for a year as a major new phase of construction gets under way.
National Highways is set to begin work to create a new underpass beneath the A47 as part of the £200 million-plus revamp of the junction on the edge of Norwich.
Work will also start on a new mile-long link road connecting the A47 to the southbound A11.
The work will mean the westbound A47 exit slip road will be reduced to one lane from next Monday (November 16), which will remain in place for a year, until winter 2027.
Drivers have been advised to plan ahead, allow extra time and, where possible, travel outside peak times.
Longer journeys are expected on the westbound A47, particularly during busy times.
Chris Griffin, National Highways project lead, said: "We are now entering a major new phase of construction as work begins on excavating the underpass on the westbound side of the junction.
"We appreciate this will cause disruption for people who use the junction regularly, but carrying out the work in this way is essential to keep both road users and our workforce safe.”
Building the underpass involves digging up to six metres below ground level.
Creating that and the link road will involve removing about 80,000 cubic metres of earth between the A47 and A11.
National Highways expects that will require around 7,000 lorry movements.
The wider improvement scheme, which includes two new free-flowing slip roads linking the northbound A11 with the eastbound A47 with two underpasses, is expected to remove about 12,000 vehicles from the Thickthorn roundabout every day.
The project also includes a new local link road to connect Cantley Lane South and Norwich Road.
A footbridge over the A47, east of the junction, will provide routes for pedestrians, cyclists and horse riders.
A number of overnight road closures are also coming up.
On Wednesday (November 18), the A11 northbound and southbound will be shut between Thickthorn and Wymondham from 8pm until 6am, for work on the new A11 bridge.
And from 8pm until 6am on Monday (November 23) and Tuesday (November 24), the westbound A47 will be shut overnight between the A140 Ipswich Road and Thickthorn junction so vegetation can be cleared and barriers put in place.
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