Depends on road surface - there are tons of steep descents around here; I do 30+ mph every morning on the way to work, a few blocks from my house, down a suburban street without much concern because I know the road so well. On bigger hills, 40+ is not unusual; I seem to start chickening out around 48mph or so because I haven't hit any max speeds higher than that.

Any of these speeds can be worrisome on bad roads -- some mountain descents have potholed roads and rocks on the road surface.