I use spackle to make my roads. First I build the basic contour from styrofoam and cover it with Sculptmold. Then I spead spackle on top with a putty knife getting it as even as I can. After letting it setup overnight I use a damp sponge to smooth it down. Now it mimics the surface texture of asphault complete with little bumps and cracks.

This road crosses the BC&SJ mainline and the spur to the grain elevator. The bridge in the distance spans the tracks before (in theory) leaping the Willamette river (modeled as an aisle).

Click here for how I build roads.