There are a lot of different types of road out there, so lots of potential answers to this. I suspect that one aspect of it isn't the road itself but what's on it. Even with fractals or noise you are still going to get a pretty similar surface over large flat areas. Real roads typically have a drip line - the darker part in the center where all the car's have dripped oil. There will be ruts , cracking, and pitting of some level at the wheel points and dust/dirt along the edges. Again how much and what kind is going to depend upon the type of road you are doing. I haven't attacked this myself, but think it may be possible to do all these things using procedural layers and the function editor. My suggestion is to pick a specific prototype and really look hard at it. Examine all the color and texture changes and how they may relate. That should give you some places to start, or specific questions you can ask.