I recently came across a video where the modeler demonstrated making roads using the 3/16" foam board product (mentioned above by a few posters) and gluing a 60 grit sandpaper to the foam board. His next step was to spray either a gray or black paint over the sandpaper. The final step was brushing a whitewash of sorts over the paint to create a weathered look.
Anyone ever try this approach to making roads for the layout?