http://udn.epicgames.com/Three/UsingDecals.html Use decals sparingly. Vertex Blending is usually a better option, but decals are a quick and good way to get small changes on geometry.
Floating planes with opacity maps and shadows turned off work as decals too. If its for rendering there are probably some material tricks you can do to combine different maps.
AH Dandi8 make your track pieces modular :P Have a look at the crappy road bits I made in my portfolio (nfollmer.com and look at the mod work). The straight piece and the curves all use the same textures. Most of the larger cracks and potholes are done with decals.