I think the key difference is whether the panel is placed at ground level (as in a solar road) or raised (as in most large-scale solar installations where the panels are raised on mounts that can also move and track the sun). The latter is pretty much self-cleaning when it comes to dry debris because the wind would eventually blow them away onto the ground (where the former will collect them forever).

I agree on both. It (degree fouling of the solar panels) is a fact of things, but shouldn't impact things much pragmatically.

I think that we can expect the same amount of accumulation of stuff as we currently get on these roads. Its not like we're out there street sweaping these rural highways, but they're also not covered in debris.