If they're already completely congested, it makes no difference if you close the road.
In this case, from town the signed route for Ashton is Ashton Old Road. Ashton New Road is signed only for Droylsden and Clayton. All it needs is a matrix sign on Great Ancoats Street saying "Event congestion on A662 - use A635 for Clayton, Droyslden, and Ashton" with temporary signage on Ashton Old Road to direct people back to Ashton New Road (
e.g. Fairfield Road / Edge Lane).
(Taxis would still head for ANR but at least we wouldn't have an artic tanker blocking traffic as we did a few weeks ago. How the driver of a 53 bus ended up on Every Street last month is still a mystery.)
And don't forget Braess's paradox.
https://supernet.isenberg.umass.edu/articles/braess-encyc.pdf
Yes, some of us have studied this stuff!