Happy New Year to you all. M.E., the road closed signs you refer to are a permanent fixture,opened by some water board official, but often neglected to close after the water returns to the river.The majority of drivers, especially in flood conditions, are local and know the road and its high and low points very well. Having worked in Llanrwst for 20 or 30 years on and off and lived there for around 8, I count myself as one of them.
From the Trefriw side of the river you can see the whole stretch of road. I have driven along there when the road closed signs were visable many, many times ---- and almost daily in a bus. Mike