I think this is an incredibly good idea, if and only if, the structural integrity, transparency, traction, glare, etc. etc. can all be properly maintained. Allthough he did mention that even operating at 15% efficiency, these roads can power the entire country annually. Imagine developing nearly-perfect, efficient roads in the more urban areas, giving us more energy output in denser areas.
http://www.wimp.com/solarhighways/
I for one believe we need to drape our skyscrapers in Thin-Film PV sheets. The cost is minimized and the area of efficiency is maximized.