See my comments in this thread about painting concrete. It involves holding a can of spray paint 3 or 4 feet above the pavement, giving a few quick bursts, and letting the paint particles settle onto the model giving a speckled look. I use a couple of shades of brown. The gray comes from a wash of India ink and alcohol.
The pic below is for a road surface made from Depron molds. If you want a smoother surface for sidewalks, you can use craft foam to make the molds, or just paint styrene sheet. The painting technique remains the same.