the road from Kinlochhourn to near Tomdoun in the Western Highlands is a 22mile long cul-de-sac. This one takes about an hour to drive because the road is so bad. According to theaa.co.uk, Kennacraig to Campbeltown is 32.5 miles (Kennacraig, not Oban, as there are a couple of turn offs between the two). Other roads may be longer but I do not have an atlas on me right now. I would guess that the one and only road on the island of Jura, or the road round the Knapdale peninsula, might be longer. If you don't count unclassified roads, then there must be some pretty long drives, like Salen to Fionphort on Mull, Lairg to Tongue on the north coast, or Kinlochewe to Corrieshalloch junction in the northwest (which would be 63 miles although there are a few cul-de-sac turn offs to villages, not sure if these are unclassified or not).