Why do wet roads look darker than dry roads?
Short answer
Rain does not dye the road. Water on and in the rough surface changes how light is scattered and reflected. Less diffuse light may return to your eyes, so a wet road often looks darker.
How it works
A dry road has tiny grains and hollows. Light scatters in many directions when it hits them, and some of that scattered light reaches your eyes.
When the road gets wet, water covers the surface and can enter small gaps. This changes the paths light takes. Some light enters the road and is absorbed. Some light reflected upward can be sent back toward the road by the water layer, where it has another chance to be absorbed. As a result, less light is scattered back toward an observer from many directions, making the road look darker.
Why can a wet road also look shiny?
A water film can reflect light more like a mirror than a dry, rough surface does. At certain angles, light from the sky, street lamps, or headlights reaches your eyes as a bright patch, even while the surrounding road looks dark.
Key point
The road looks darker because the amount and direction of light reaching your eyes have changed, not because its color has changed. The appearance depends on the surface, how water is spread across it, and the positions of the light and the observer.