Unsuspecting motorists face a near 12-mile diversion when a key but small stretch of Saddleworth road is closed for retaining wall reconstruction.
Work on the costly Standedge Road/Acker Hall Farm project in Diggle, near to the A62 Huddersfield Road junction, is expected to last six months.
Contractors were due to begin repairs, estimated in excess of £400,000, as the Independent went to press. They are scheduled to last until January 2019.
Oldham Council ruled out 24 hour working to shorten the length of repairs on cost grounds.
There have been temporary lights at the site since early June.
But a full closure of the A670 is now set to test the patience and Sat Nav programming skills of drivers unfamiliar with the area.
Highways bosses agree local knowledge will prevent many embarking on the full alternative route. Others, including HGV drivers, might not be so lucky.
Any vehicles travelling along the A62 from Marsden will be unable to turn left towards Uppermill, except during Yanks weekend (August 11-12), and Rushcart celebrations on August 25.
Instead, they will be directed along the A62 to Cross Street in Oldham Town centre before being turned back towards Saddleworth along the A669.
An original shorter diversion route was set to take traffic down Platting Road at its junction with the A62 and Doctor Lane to the Lydgate crossroads.
But that has been scrapped and confirmed by Gary Sutcliffe, Principal Engineer for Unity Highways.
“Further to concerns raised by the wisdom of using Platting Road due to the physical layout of the junctions at either end and the overall condition of the road itself, the route has been amended,” he said.
“With regard to the suggestion the contract period could be reduced by implementing 24 hour working, this has been looked into and rejected due to significantly increased costs, the safety of workers due to night time working and the quality of life of nearby residents.”
The existing wall, supporting the A670 and which first collapsed in October 2014, will be fully demolished and reconstructed.