Drivers using the Bromley Heath Viaduct face serious congestion because of resurfacing works.
From Monday 24 July the Viaduct - part of the busy A4174 ring road will be reduced from two lanes in each direction to a single lane in each direction.
Work will last 33 weeks and could cause up to three miles of tail backs.
South Gloucestershire Council says the maintenance will make the road safer for drivers, cyclists and pedestrians by updating the paths. The extent of the work requires a section of the southern dual carriageway to be closed to all vehicles. This is because the viaduct will be too weak to safely carry traffic until the maintenance is complete.
A spokesman for South Gloucestershire Council said: "We apologise for the inconvenience caused."
There are also closures to some local roads, plus adjustments to traffic lights and road signs in the surrounding area.
Traffic heading to the M4 will be directed East to junction 19 of the M4 rather than using junction 18. Signs are in place to alert drivers.
The Council are encouraging people to think about changing their travel plans to help ease congestion, especially at peak times.
There will be extra buses available on the number 18 during peak travel times.
A park and ride facility will also be in operation at Lyde Green, in Emersons Green.