You are not factoring in the added savings on asphalt. Asphalt roads are expensive. For example, the new I-4 expansion in Orlando will cost $2.3 billion for 21 miles of 4 added lanes. The reason solar roadways make economical sense is because it replaces the road itself and thus adsorbs the cost of the asphalt that would otherwise have to be used. Solar canopies only make sense for parking lots and only in hot regions.
If there are any materials you could choose to use that are vastly more expensive than asphalt, glass and solar panels are right up high on the list. The price of a solar roadway would be staggeringly higher than a regular one. (Currently, most of the cost of roads is labour, earthworks, concrete piling, steel structures, etc.).
But solar roadways are ridiculous for another reason - first, putting a solar panel under a thick layer of glass will lower its efficiency greatly - because the thickness of the glass will reduce light transmissibility through it, making the panel far less efficient than a regular one on a roof etc... Secondly, the glass is really slippery, and the only way to fix that is by texturing it or coating it in something, both which will reduce transmissibility even more. Next, if you've ever dropped an ice-cream on the road and the picked it up again, you'll know that roads are ridiculously dirty. The dirt and grime will build up, just like regular roads, reducing the efficiency even more. Then, the cars driving over it will grind that dust into the glass, and any abarasive material will put tiny scratches in the panel - which over time will build up and again reduce efficiency further.
Dave Jones from the Electronics Engineering Video Blog (EEVBlog) did some calculations in one of his videos - assuming optimal efficiency. Even then, a panel would not make enough electricity to pay itself back (can't remember the time period exactly - over 20 or 30 years? Or perhaps it was the estimated lifetime of a panel).
But with all these extra losses, there is no way it would ever work without huge advances in solar technology and materials science.
Why yes they are. You need more than just solar panels though. You'll need thick tempered glass, power inverters, monitoring and management systems, transformers and high voltage power lines. Once installed, your glass blocks will provide the ride comfort, noise, safety, of brick or cobblestone roads but less durable and more slippery.
I seem to remember that cobblestone roads were almost as slippery as ice whenever it rained. I grew up in a town that actually had cobblestone streets (long story, Europe).
