I thought of this idea when I was a kid on a car trip. I could probably even find my sketches to prove it. Now some Japanese researchers have built a musical road powered by grooves cut in the pavement. At the time, I didn't want to play music, rather I was imagining these things as advertisements. Seeing the video that's linked, though, I doubt you could get enough clarity or resolution to make that happen.
The other idea I had for it was even more annoying. It would be possible to play the same tone (A 440 MHz anyone?) no matter where you are. That is to say: The spacing of the grooves would be adjusted so that when you're driving at the speed limit, it will play the same tone. A speed limit for the perfectly pitched!
