A landslide caused by Storm Darragh means what was a six-mile trip is now 25 miles.
The A487 has been shut between Corris and Minffordd, Gwynedd, since Wednesday because of worries about unstable ground above the road.
The Welsh government said it hoped to resolve the matter quickly but there are fears it could be closed for months.
Builder Martin Jones, who was driving to Dolgellau just after the landslide, caused by Storm Darragh on Saturday, said: “If I was 10 minutes earlier, I could have been caught up in the landslide and, you know, I’d have been stuck or maybe worse.”
