Considering the time of year i.e peak season the side roads were very quiet.
I think that's the advantage of SatNav's as the route from Llangollen to Festiniog doesn't take you over the B4391, purely because it starts with a 'B'.
https://www.google.co.uk/maps/dir/Bl...66!2d52.969215
The better route :
https://www.google.co.uk/maps/dir/Bl...d52.969215!3e0