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Some highways in the US have a warning patch on the shoulder. It is a corrugated surface that vibrates the vehicle when the tires roll over - it makes a sound too. What if a modified 'corrugated' surface was placed on the road. This surface would have been carved using a special computer controlled machine, essentially 'Laying Down a Track' similar to the old Vinyl Records. The computer would use the average speed of the highway/road to lenghten or shorten the track to make the car hear it clearly.