During the Fall of 2010, I worked on some roads on my layout. Paving roads has been covered several times in the hobby press, everything from asphalt roofing tiles, to styrene has been used.
I saw an article (forgive my forgetfulness on where) online, that suggested using Durham's Water Putty. This product is typically used for patching holes in woodworking, or walls.
To begin, make a "mold" out of some Balsa wood. I use Balsa, because it is soft, which makes it easier to remove after. I typically make my mold, in place, on the layout. The road width varies, depending on what you're modeling, for me, it has 12' lanes, for a total of 24' wide in HO scale.
Mix up a batch of the water putty, making sure it's fairly watery, like a thick soup, not peanut butter. Pour it into the mold, and squeegee the top to level it with the sides of the mold. After about an hour of drying, you should be able to remove the molds, make sure the surface is not soft before you do so!
After allowing it to dry for 2 days, I sanded the top lightly with some 300 grit sandpaper. Vacuum off the dust, and then paint. I used some Acrylic black by Testors. This paint is fairly thick, allowing you to paint the road in one coat. I painted the sides as well.
Once the paint has dried, I build up the sides with some scrap material, and cover it with gravel. The painted edges of the road can be left exposed a little, to make it look as if the pavement is on top of the gravel.
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