The way I would texture a road within a given terrain is with a seperate mesh and a horizontally tileable texture such as this:
Then make sure the texture on the edge of the road and the terrain bordering the road looked kind of similar. That would however always create a visible seam such as in the image below taken from half life 2 episode 2:
Now the same game seems to blend in a horizontally tileable texture (the road) with a terrain made up of horizontally and vertically tileable textures (grass & dirt). How is this possible without a seperate mesh/material and without any visible seams, especially since the road is twisty meaning the texture has been rotated. I am completely baffled as to how this has been created, this is clearly beyond standard blend maps.