A five-week maintenance project on the A38 Marsh Mills viaduct, near Plymouth, is due to start next month.
The scheme, which starts overnight on January 6, requires the westbound carriageway over the viaduct to be closed while one of the structure's expansion joints is replaced.
A contraflow along the eastbound carriageway will be set up and a temporary speed limit of 40mph imposed.
Highways Agency project sponsor Alexis Field said: "We will do everything we can to complete this essential work as quickly as possible and specialist teams are ready to work seven days a week.
"The expansion joints allow the structure to expand and contract with seasonal changes in temperature. Surveys on the Marsh Mills viaduct have shown that the joints are reaching the end of their serviceable life.
"The work has been carefully planned to ensure it causes the least possible disruption to road users."