Drivers are facing a 20-mile (32km) diversion on the A30 in west Cornwall while road resurfacing takes place over four nights.
The Highways Agency said it had to close the road between St Erth and Crowlas to strengthen the road surface.
Diversions will be in place between 2100 and 0600 BST until Thursday, but the route could take up to 40 minutes to complete.
Some businesses fear they could lose out while traffic is diverted.
Peter Heron from Highways has apologised for the inconvenience, but said the work would prevent lengthy road works in the future.
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