Temporary lane closures will be in place on the approach to the Stoneymollan roundabout, Balloch, as well as the nearside lane on the roundabout.
Work begins on Monday and is expected to last around two months.
Scotland TranServ, the operating company for south west Scotland's roads, will carry out the "enhancement work".
Lane closures will be in place continuously throughout January from 6pm on Sundays until 3.30pm each Friday.
The layby on the north- bound approach to the site will be closed throughout and advanced warning signs will be in place to alert motorists of possible delays.
Russell Rennie, Scotland TranServ's contract director, said: "We are delighted to work in partnership with the National Park to deliver this exciting project.
"The planned improvements will give the roundabout a much needed facelift benefiting road users, local residents and visitors to the area for years to come."
The road - dubbed the Gateway to the Highlands - will see improvements to its drainage and landscaping.
And the landmark Flying Geese sculpture on the roundabout will be infilled and painted white.
The traffic management measures will be removed in early February, while the white geese are being refurbished.
They will be reinstated during early March for the remainder of the works.
Flowers and trees will be planted and a maintenance access road will be created to complete the project.
Scotland TranServ bosses said they will work closely with Transport Scotland and the National Park throughout the project.
For more information, visit www.scotlandtranserv.co.uk