Black mountains is an awesome driving road. Did it once at 7pm on a winters night in a renault luton van. Thickest fog i've ever experienced, met a car on a really narrow part, moved over to let it past, got stuck on the grass. My work buddy offered to get to push but feared he my fall 300ft to his death!! All good in the end though, the little old lady in Llandeilo got her headboard and no soiled pants. honest!!