I've used sandpaper for roofs, certainly. Works good.
Less thrilled about using it for roads or parking lots. Scenery work (at least the way I do it) tends to use a lot of water, so any sandpaper would turn to mush most likely. Plus it's just too easy to texture a road or parking lot with a more realistic variety of textures than just what sandpaper would provide.
It's the subtle variations in texture that really makes your scenery realism pop. Sandpaper would be too uniform in many cases for roads or gravel parking lots, where tire ruts and bare spots will really spice up the realism factor.
In place of sandpaper, I use fine sand or fine ballast, spread it out dry on the road or parking lot, wet it well with 70% isopropyl alcohol, then soak it with white glue diluted 3:1 with water (and a couple drops of dish detergent thrown in so it flows better). I'll come back in with a brite boy and burnish out the ruts or add bare spots. Finally I'll hit it with my plaster-tempera weathering powders as described elsewhere in this forum thread and on my layout web site.