Or just put them above the road then? :P
Under the road is literally the only option, and it's a terrible option at that. Rooftops - or better yet, former farm fields- are always going to be cheaper and more efficient.
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.)
https://www.cbs58.com/news/truck-wedged-under-bridge-shuts-d...