Article: Solar roads are more practical than they sound

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5. I wanted to know about grease and oil from cars and how that would be cleaned off.

From the FAQ:

"There is a very common natural element called titanium dioxide, which turns substances like oil and grease into a powder that would be blown off by wind or washed away by rain. It's currently used on building facades to keep them clean. Spraying a road surface with titanium dioxide or a similar coating may solve the problem."

Only problem is titanium dioxide (a molecule, not an element) is classified as possibly carcinogenic to humans, not something that should be sprayed throughout the urban environment.
http://www.ccohs.ca/headlines/text186.html