We didn't get that far. We figured out what size/spacing you would need for the frequencies of intelligible speech given a tire moving 60MPH, saw that it would take something very hard (like steel) to do it, and that that would be very expensive and also not provide any sort of traction, and stopped.

The idea was to replace the toll booth strips with ones that actually said "slow down!" we figured if you were already going slowly, they wouldn't be intelligible.