I spent a fair amount of time doing model railroad groundwork, so I didn't want to spend a lot of money on comercial hobby materials... I use sifted dirt from the local baseball infield, a nice grayish-brown color that "scales down" well, mix with water and white glue to make mud, then applied it with an autobody spreader. It dries the right color, since it's dirt already... I can work in ruts and tracks, then give it a quick dusting with baking soda mixed with powdered tempra paint. I lock it down with a water/white glue/dish soap mix from a spray bottle. After it dries, I give it a few coats of clear flat varnish.
For Western European streets, I made a "cobblestone" mold out of a couple Tamiya brick wall sets by glueing them all together, making a male mold out of RTV (Room Temperature Vulcanizing) silicone by appling it over the plastic bricks, then cast as many sections as I want in plaster. I bounce a steel cutter in the Dremel tool at random spots to chip and gouge the plaster. Seal it with a clear varnish, apply the base colors, then apply washes and dry-brush. Lock it down with clear flat.
For pavement, I use sheetrock painted in a greyish-tan and add tar-caulking lines with thicked acrylic dark grey-browns after I scribe them in. Gravel roads are playground sand you can get at any home-improvement store. A 10 pound bag will last a lifetime, but I just sweep up what's laying around on the ground by the pallet and put it in some ziplock bags. (I always keep some ziplock freezer bags and a large 4" paintbrush in the car in case I spot some "ground material".) I just paint the base with white glue and sift the sand onto the glue. After it dries, I shake off the loose sand, then sprinkle more sand onto any bare or thin spots, and lock it down with the water/white glue/dish soap mix again. (Always use a few drops of soap in the mix. It breaks down the water's tensile strength and allows it to soak in fast, rather than standing up in glob on top of the material. Don't shake it to mix though, or you'll have a bottle of suds)
Don't forget to carry the ground colors up onto the vehicle's tires, tracks, etc, so it "ties" the vehicles to the roads. Unless you're doing a straight display, that is... Then it doesn't matter since you're not building a diorama.
Another thing you might consider for roads is to build on top of fiber ceiling tiles, the 2 ft x 4ft variety. By using those, it's easy to make a raised road-bed with ditches. Just glue them to your base in three sections, then landscape with your ground materials. I use construction adhesive to glue the tiles down, but white glue will work. Glue them with the finshed side down. In small scales, if you break the tiles rather than cut them, they make great layered rock textures.
Anyway, this works for me when I want a large stretch of road/street, say anything over 3 feet... They would also work for the surface of the display shelf itself if you don't mind it being permanent.