Ah, Gorilla Glass. I just looked it up, and it is orders of magnitude better than regular glass. In terms of hardness, it's a tiny bit softer than quartz, and apparently it's actually not too expensive to produce.

Still, all sorts of rocks and gravel can end up in tire treads, and while they'd likely last quite a while, they'd nevertheless build up scratches over time, reducing efficiency along with anything else that ends up reducing exposure to the panels, on top of the already ~60% decrease in efficiency from them being laid flat in the ground.

Not only does the glass have to be as tough and durable as asphalt, it has to be tough and durable enough to stay transparent long enough to give a ROI on whatever the cost of both the glass and the panel itself is.