Last week's answer

Question: Along a stretch of historic Route 66 near Tijeras, New Mexico, highway engineers installed a unique traffic-calming feature: a "musical road" where driving over raised rumble strips produces the opening notes of America the Beautiful. How did engineers tune the asphalt strips so that a car's tires play the correct musical pitch as it rolls over them?

Answer: By calculating the exact physical distance (spacing) between each rumble strip so that a car traveling at 45 mph hits the grooves at precise acoustic frequencies.

The musical road was installed to address an accident hotspot along Route 66 as a traffic calming measure, as it encourages drivers to slow to 45 mph to hear the song. 

Sources: Route 66 Musical Roads