There's a standard maximum height for vehicles that is taken into account for freeway bridges, so your first argument is nonsense.
The rest still stands, of course. If you absolutely do want to put solar panels in the same place as cars, starting with parking lots is clearly a better idea. Fewer problems with high speed collisions, the electricity is generated closer to where it's needed, and people will be thankful for having parking in the shade. (Still more expensive than putting panels on a roof or field of course.)
"Nonsense" would only apply if practice were as good as theory; a very very brief Google search will net plenty of results for freeway overpasses and pedestrian bridges being hit. One example:
This isn't really a criticism of the core idea to just say "what if somebody built it stupidly low", traffic lights have been fine my entire life and maybe some highways will have to be skipped or doubled for NASA/ military purposes.
The rest still stands, of course. If you absolutely do want to put solar panels in the same place as cars, starting with parking lots is clearly a better idea. Fewer problems with high speed collisions, the electricity is generated closer to where it's needed, and people will be thankful for having parking in the shade. (Still more expensive than putting panels on a roof or field of course.)