Some people on youtube did a bit math and simple home experiments on the economy of paving roadways with solar panels.
https://www.youtube.com/user/Thunderf00t/search?query=solar
https://www.youtube.com/user/EEVblog/search?query=solar+road...
https://www.youtube.com/user/Thunderf00t/search?query=solar
https://www.youtube.com/user/EEVblog/search?query=solar+road...
Horizontal placement, dust, scratches vs brittleness, mechanics of moving load bearing by tiling vs continuous surface, cost of PV turn it into a joke. We'd be better off putting them above the roads than on the roads.