I got several techniques...
Curbed roads
If I am doing roads with sidewalks or curbs, I would build the curbs first out of stick styrene. Typical curbs would be about 6 inches wide and 6 inches taller than the road surface, so I would use something like .100"-square styrene strips to make the curbs (which outlines the areas you have to pave). Then you spread the drywall compound between the curbs in a 1-mm layer, and pave your road. Voila. :thumb:
If you want to go nuts with the detail, you can even put in storm drain grates (you can take a small piece of rectangular brass or sheet styrene and drill #65 drainange holes in it, paint it up in a rust brown color, and embed it into the drywall compound road surface next to the curb). Ditto for manhole covers-- Use round pieces of brass or sheet styrene about 10mm wide (typical manhole covers are about 2.5 feet in diameter I think) and drill some #80 holes in it.
Rural paved roads with no curbs
If you are paving a rural road (no curbs), just temporarily tack two strips of .040" styrene (.040" is about 1 millimeter thick) to delineate the areas to be paved, the start spreading the compound between it. After the compound sets, take away the styrene strips. Your road is paved.
Roads with drainage ditches to the side
I don't see these much around my area, but I suppose you can use 1/4" U-shaped styrene channel (Evergreen and Plastruct sells these). 1/4" works out to be just under 2 feet wide in HO. Recess these into the foam base with 1mm sticking above the base to delineate the road, then pave.
The common thing is to make the road pavement about 1mm thick. Any thicker than that, drywall compound takes too long to set for my tastes.
Good luck!