Why can distant roads look wet on hot days?
The Real Answer
Distant roads can look wet on hot days because hot pavement heats the air immediately above it more than the air higher up, creating layers of air with different refractive indices. Light from the sky and distant surroundings bends as it passes through this strong temperature gradient, with the rays deflected toward the cooler air.
Your visual system normally assumes that incoming light traveled in a straight line, so it traces those bent rays backward and places the apparent image below the road surface. That can make blue skylight look like light reflected from a puddle. This effect is called an inferior mirage: the road is dry, and the apparent water is an optical image produced by refraction through the hot, uneven air above it.
Nothing makes you distrust a perfectly dry highway faster than a puddle that keeps moving away from you. You know it’s a mirage, but you still watch the car ahead to see if its tires throw up water.
Why People Ask This
People ask because the effect can look convincingly like a real patch of water from a distance, then disappear as you get closer. Seeing what appears to be a puddle repeatedly retreat down a completely dry road makes the visual effect especially noticeable and confusing.