The B769 Old Glasgow Road from Stewarton to Newton Mearns has shut for a month for "essential works" and repairs to Corsehouse Bridge, leaving commuters and local drivers frustrated.
It means commuters, thousands of whom use the link road every day for travelling into Glasgow for work, must take a detour to Fenwick to join the main road, the A77, leading into the city.
The road had previously shut for 12 days in July for resurfacing.
Drivers said the move was a "major inconvenience" and questioned why the repairs would take a month.
One Stewarton resident, who asked not to be named, said: "This is a real inconvenience.
"I use the road to get to work in Glasgow. It now means I have to take a long detour.
"I appreciate the bridge has to be repaired, but does it really have to take four weeks?"
Councillor John McGhee, who covers the Annick ward on East Ayrshire Council, said he would be investigating.
He said: "I know there are a lot of people who commute to Glasgow and that it is sometimes easier to use that road because it is less busy, so I can understand there may be issues."
A council spokeswoman said the work was essential in the run-up to winter.
She said: "Work has begun to repair the partial collapse of the parapet of the bridge.
"Other essential works will be carried out at the same time. The council apologises for the inconvenience this may cause."