Safety work on a viaduct on the A38 in South Devon will start next month.
The scheme on the Mardle viaduct, near Buckfastleigh, involves re-waterproofing the westbound carriageway. Work will start on November 5 and last for six weeks.
While it is taking place one lane on the viaduct will be closed 24 hours a day. The remaining lane will be narrowed to allow the work to be carried out safely. A 40mph speed limit will be in place.
The adjacent slip roads on to the viaduct from Dart Cross bridge and Peartree junction will also be closed to allow traffic to flow smoothly through the one lane.
Roger Stribling, from the Highways Agency, said: "Users of the viaduct will appreciate that we occasionally have to close the carriageway to carry out routine maintenance and repairs to tackle wear and tear.
"We will do everything we can to ensure that disruption to road users is kept to a minimum, but drivers should allow slightly more time for journeys. The work being carried out is very weather dependant because the waterproofing cannot be applied while it is raining."