Distance Map is the actual physical distance, rather than just height, so as things get further from the road in the horizontal, the fog will also fade out, as well as with how far from the vertical. I am not sure I am understanding correctly, but if it is just height you want, I'd place a plane larger than the scene, set it to non-renderable, and use that as the Distance From object, which will give fog that fades at a given height across the whole scene, effectively making it a "height only" effect.