While working on the figures, I also work on the simple "landscape". I have at another occasion bought a series of bases from a craftsshop, and one of them was slightly smaller then the one I plan to use, so it could be used as a template for the street.
Bottom layer is foamboard for its ease of working, is also quite stable so that it wont warp when painted or glued, which makes it ideal for this kind of work, where the upper layer of cork is glued on with white clue and later treated with wall filler. I have used a sheet of balsa wood to lift the sidewalk area compared to the road area that have just received a piece of cork.
Cheers/Jan