Weatherproofing the camera and solar panels, getting permission to get them installed, actually installing them (working at heights over busy roads) and then maintaining them would be pretty expensive. You might also have to pay rent to whoever owns gantries, and what about locations where there aren't any existing gantries?
What about insurance in case you camera falls off the gantry onto a car and kills someone?
Edit: What about roads that don't have network coverage (not that uncommon in the Scottish Highlands)