Instead of building giant, needlessly complicated glass roads that , even if they work, would be only mildly more efficient than regular roads while costing hundreds of billions if not trillions of dollars to implement on a large scale, why not just put solar panels over parking lots? Or invest in green roofing in cities? Or put the solar panels off to the side of the road?
This isn't the Apple 1 or the moon landing. It's the nuclear bazooka, or 3D movies, or the flying car or the jetpack. The idea is in and of itself, dumb. The Apple 1 at its core - a computer in a smaller box - was a good idea poorly implemented because the implementation was still new. Computers were already known to be useful tools. Small boxes that we can put on desks have been useful for ages. Putting some good thing in to another good thing is like 99% of all technology ever created. Even fucking rockets are just fuel put inside a really big thermos flask at their core.
Solar roads are putting two things that are bad at being roads together - glass and solar panels - and expecting them to be good roads.
Not to mention that I bet making all that glass is going to use up a lot of energy.