The investment, alongside a scheme to enhance electronic signage on overhead gantries at Kennford, aims to maintain the safety, resilience and reliability of this busy Devon route for many years to come, the agency says.
Carrying up to 24,000 vehicles daily in each direction, the A38 is a vital route for the Devon economy.
The first project, to replace expansion joints and improve drainage on the Whitecleave and Mardle viaducts will start in September and is anticipated to complete by the end of January. National Highways is carrying out a full refurbishment and replacement scheme to improve the long-term reliability of the viaducts and prevent future maintenance closures.
Route manager Ed Halford said: 鈥淢any of the structures we manage were built over half a century ago and now need renewing or replacing to ensure they remain fit and safe for purpose.
鈥淚t鈥檚 essential that we futureproof the viaducts now, to ensure their reliability and to limit the need for future maintenance work, and along with the other works, and further renewal plans for the A38, this represents a significant investment in this busy route."
The drainage improvements further west between Ivybridge and Lee Mill will address a flooding hot spot in the county and improve safety on a section of the A38 where surface water has been evidenced in periods of heavy rain.
The scheme is due to start in November, with a contraflow in place on the dual carriageway until the following October, with further details to be communicated once finalised.
Tree clearance work along the A38 is ongoing and the resurfacing scheme further east, on a two and a half-mile section of the eastbound A38 between the Peartree and Goodstone junctions, will follow in 2027, after completion of the viaduct scheme.