Don't want to rain on his parade, but I don't think he's really thought this through.
I think a highway of solar cells would be useless. How would you get the power from the highway into the local grid? There's a reason power is transmitted from the plant to the city over high tension lines - it's more efficient. It would be impractical to simply have wiring embedded into the roads taking the power from each section of solar cells and passing it along.
Think of how solar farms work. You have a lot of solar panels in a small area. Panels are grouped together to increase voltage and current. These groups of panels would send the power to a central hub where it would be converted into high voltage AC to be transmitted to where it's needed (say a nearby factory or city).
Now imagine taking that solar farm and turning it into one long solar cell array (a highway). Now instead of having a cluster of solar panels with short, low loss efficient wiring to the central hub you now have those panels stretched over several miles. That is a very in-efficient way to connect solar cells and transmit power.
Now solar roads in your own neighborhood would make more sense. If your local street was a solar street then you could get power from the road to your house and the distance the power has to travel would be very short. The problem, though, is that a street of solar cells probably couldn't generate the power the houses along it would consume. But it could reduce the amount of electricity the houses take from the grid.
Then again, why a solar street when I could simply have a solar roof? A roof collects far more sunlight since it's at a better angle in relation to the sun. Streets, being flat, are already at a disadvantage in terms of how much sunlight they get. Why spend money on expensive solar cells, and then not even bother to have them aimed at the sun to get maximum exposure?
I'm rambling a bit, but I just see so many problems with his idea.