Biggest downside to solar roads, cycle paths, crappy art installations, is simple.
It's a lot cheaper just to mount the solar panels on someone's roof, plus you get more power as they are properly orientated to face the sun.
Less travelled roads tend to be dirtier, as less cars to blow the dust from the road surface.
EEVBlog has several videos on why solar roadways are a scam, they work but as still a waste of money.