Motorists between Bristol and Bath face nine weeks of delays from Monday as the A4 closes for roadworks.
The closure, at Saltford, is part of a £1.3m maintenance scheme to rebuild the road and carry out drainage work.
Traffic heading towards Bath will be diverted off at the Keynsham roundabout and will take the Avon Ring Road, the A420 and A46 into the city.
Motorists heading into Bristol will be diverted off at the Globe roundabout and will take the A38 and A37.
Local residents in Saltford will still have access to the village using passes.
"The existing carriageway has reached the end of its life and the substructure of the road now needs to be rebuilt," said Dave Sledge, Highways Agency project manager.
"We will also be laying a new low noise surface, replacing drainage and damaged kerbs together with the resurfacing of the adjacent footpath.
"Water and gas companies will also use the opportunity to re-lay extensive lengths of their mains, reducing future disruption."