It appears the roads have no function in the game? Now that I think of it, I cannot remember them being mentioned in the manual.
There are two regions adjacent to Savannah. One has a road running from Savannah, and one does not. I noticed that starting from the Savannah region, if I move a unit into the neighboring region with no road, it takes my force exactly the same amount of time to march there (14 days) as it does to move into the region that does have a road.