I agree with Vulcan, I think your paint problem may be because the plaster has not dried thoroughly yet. Although it seems dry on top, moisture will continue to creep up until it is finally cured. Depending on the humidity in the room, it could take days.
I used a dark gray acrylic paint first and brushed it on. Then I took a lighter gray, poured a puddle of it in a plastic lid, and touched a slightly dampened regular household sponge in it, and dabbed the road surface with it. It looked so real, and you can vary the degree of the lighter gray by touching it more than once in some areas. If you want newer looking roads, use the methods Gregg or fctthlw mentioned, and use a darker gray with the sponge. Jim Barrett was very impressed with the realism of it when he was here. If you mess it up, paint over it try again.
Mike