I just posted an article on ecorenovator the other day. The main cost of a solar install is not the panels, its the supporting structure and electronics.
Alternative energy costs are dropping - EcoRenovator
I do agree with Oil Pan. Roads are already really expensive, and solar is not cheap. I'd rather not combine the two. I'd MUCH rather see them on building roofs where they're out of the way and just chugging away daily to make energy.