Roads don't add new polygons, they can only impact the dynamic performance, i.e. slow down the creation of terrain tiles. As usual, it's most demanding when you are close to the ground, roads further from you aren't rebuilt that often.
There's also one rare case that can happen with the roads (when road intersects with itself), because of which the roads aren't drawn as effectively as they could be. My guess is that once this is handled, it will hide 10x increase in roads density.