Roads are made via two different methods. The terrain painter in the world editor creates the terrain type (provides the physics), so you must decide which type of dirt you want the driving to be (mud, dirt dusty, gravel, etc.). Then paint this on the terrain. The road decal is then laid on top of this to give the markings of the road. For some roads (more commonly in the newer maps and vanilla maps, this is done using a variety of tyre mark decals in the middle, with edge decals that transition from one terrain type to another (e.g. dirt to grass, or asphalt to grass, etc.). This allows the map maker to decide the width of the road, and reduces the artefacts sometimes seen with repeating decals. I am a lazy modder though, so I use a single repeating road decal for the full width. Look at the Somerset UK map as an example. I use full width dirt track decals placed on dirt dusty or dirt loose terrain. I find this works best for me.
