Thank you for bringing this up. It's not something I've thought of as a problem before.

It does in fact - based on my 5 minutes of research - seem that it can be an important factor, especially if the panels are placed within "splash range" of the road.

When I mention solar panels near motorways I'm not picturing them right next to the road, I'm thinking of larger strips, perhaps 30-100 meters from the road and in areas that have already become unattractive due to noise pollution. There are many such areas.

I think the main issue with using them is that there are many land owners involved. It's easier to get fewer land owners to commit larger fields, than many land owners to commit small strips. But that is IMO a solvable problem, not a good reason for placing the panels next to where people like to live.

I'm betting the effect is a rounding error compared to road salt build up.