I use a wet sponge to smooth out the road surface. After the sponge treatment has smoothed the surface out, check to see if there are unrealistic holes or cracks. If so use a little spackle to patch them. When the patches dry sand them and repeat the sponge treatment. I like to use an airbrush and Poly Scale paints to color the road.
Grimy black is a good color.
Dilute it a bit more than usual with air brush thinner and build the coloring up gradually. The center of each lane should be darker from grease and oil dripped by passing vehicles. To darken areas try a bit a VERY diluted steam power black. Use oily black to simulate a place or two where a car or truck really unloaded. If the road has multiple lanes and will be striped, be sure to add the stripes before completing the weathering treatment.