Everything has a drawback, but covering roads and highways panels would be frequently equally as prone to damage ( every time there is a downed tree or fender bender). As for dust, the exhaust fumes of 100,000s of vechiles adds up, and the panels cannot simply be hit with compressed air. The soot sticks to surfaces.

Its nice in theory but actually covering highways in panels would make them disgusting choked tunnels and ruin the panels. Covering the highways at all even with a light canvas to block sun would be prohibitively difficult to make both effective and maintainable.