More information on this: (I've been trying to figure out the road algorithm in my Iroquois game)
Something that's rather counter-intuitive about the forest roads: although they count as roads when you attempt to enter the hex, they don't count as roads when you're leaving. So if your "road" goes from a forest (without a road built on it) to an unforested hill (with a road built on it), it costs your unit 2 movement points; as far as the game is concerned, you just moved from a hex w/out a road to a hill, which always costs 2 movement points.
So could you circumvent this problem by building a road on the hill tile and the forest tile before it? Would that then make the road continuous?