Can only concur with what's been said about CA36. It takes it's place among only a few roads that my SO and I have ridden that have caused us to quit for the day at their end. We rode from the Napa Valley over to CA1, up the coast to Rio Dell and 36 to Red Bluff. When we got to Red Bluff we noticed a motel across the intersection and, without a word, both headed for the motel parking lot -- enough for today. Actually, on the east end, the road gets into a kind of rythm that encourages you to look forward to the approach of the next corner and successfully dispatch it -- I think that's what tends to (sneakily) fatigue you (or, us, anyway).
Amother road like that has been ID3 south of Coeur d' Alene, east of Moscow, and the back road, P1, into Orofino. We've actually done that twice -- repeated it years later to verify, we really were that tired the first time. Yep, it was the road.....