I am using a technique I learned by watching videos by Luke Towan and Marklin of Sweden by using cork or extruded foam to form the road.  This road bed is then covered over using plaster of Paris or other plaster or gypsum product using a small trowel or simply using your fingers.  After fully drying, the plaster is sanded smooth and painted an earth/dirt color, readying it for the application of dirt texture.

The dirt texture is made up of real dirt from the back yard dried in the oven and sifted for size and sanded tile grout purchased from a home improvement store. This is applied by first brushing on a mixture of white glue and dirt-color paint, and then “sprinkling” or shaking on the dirt/grout mix through a screen (I used an old nylon stocking), ensuring a thorough coating is applied.  Don’t worry about using too much or it landing on something other than the road, the dirt will only adhere to what was painted with the glue mix, the rest can be brushed or vacuumed away after drying.

The key to making the roads look realistic and dusty is the application of weathering pastels which are brushed on the areas of the road that tires would normally run on, feathering out the pastel dust towards the edges and middle of the road.