Work is beginning on a 14-week scheme to upgrade drainage to stop flooding on the A27 in West Sussex.
The Highways Agency said the two-mile section of road between the Lancing College junction and Dankton Lane, Sompting, would be affected.
"The drainage system along there is quite old and we have had some failures," said manager Peter Phillips.
"We need to update it as well because of the changes in rain patterns we are seeing these days."
The first two weeks of the project will involve clearing vegetation and some excavation, with the main roadworks starting on 1 October.
Flooding has caused deterioration to the surface of the road, used by 56,000 vehicles a day, the agency said.
Work will be carried out seven days a week during the day and night, but avoiding peak hours.
"There will be some disruption," said Mr Phillips.
"We've worked very hard at looking at the traffic management but if drivers can adjust their journeys or allow a bit of extra time that would be very advisable."