It has to do with the surface the available light is reflecting from. Our eyes capture and measure light, then send that information to the brain in the form of electric nerve impulses. Those impulses the brain interprets as images. The surface of the road ( I am assuming you are talking about a blacktop highway, made out of asphalt) normally has a rough texture to it, which scatters and absorbs light. When the road is covered with rain, the water provides a smoother surface, which reflects more light. But the light does not penetrate the film of water on top of it, so it is harder to see what lies underneath. That is the primary reason it is so much harder to see the road at night when its wet. When the road is dry, there is true reflection from the surface of the road, which makes it easier to see. Other factors that might make a difference would be headlight quality and the presence of fog, or steam coming off the road, which would tend to reflect light as well.