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Thunderfoot covered it. Doesn't work. You increase your road infrastructure complexity by too much. There is even better lower hanging fruit (parking lots, building roofs), and until you fully saturate that lower hanging fruit it doesn't make sense.

Remember, returns on solar are tight. You want to spend as little money on mounting it as possible. Building pylons that can withstand a car crash, putting those pylons as far away from the road as possible to reduce driving hazards (even if they can withstand a car you almost don't want them to because that could kill people), then building a roof to span all of that where there wasn't one is not minimal mounting costs. There are places where some of those factors we want already exist without additional construction.

Of course some will say, but we make pylons like that all the time. Yeah, for expensive overpass infrastructure. Not for every mile of the road. You want to keep your standard road as cheap as possible, and you want to keep your solar mounted on the low hanging fruit all around you.