Asphalt is very dark in color and therefore absorbs most of the sun's energy as heat. If you've every tried to walk barefoot on a hot summer's day across blacktop you can certainly testify to the thermal energy present.
I saw another post recently that proposed making roads out of photovoltaic cells.
I think this is a step in the right direction, but the cost of production, the low efficiency, the frailness of the devices, and the need for direct exposure to the sun's electromagnetic waves is very constraining.
So I propose to use Peltier devices, or the Seebeck effect to be more specific, to generate power by using the temperature differential between the asphalt of the roadway and the underlying bedrock/clay.
You've probably heard about peltier devices and know that they are usually made of n-type and p-type silicon. This makes them costly. But with the vast amount of roadways in the US alone, efficiency could take a hit by a factor of 10 or more from the commercials peltier devices sold, and still generate enough energy to be comparable to today's US power consumption.
I imagine with a little research by materials scientists, we could come up with an environmentally friendly, inexpensive, and large-scale peltier device for use underneath the roads of the future.
Power to the people!
I've included some links you can check out to learn more about the Peltier, Seebeck, and Thompson thermoelectic effects. :) enjoy.