Screw it. Let everyone put signs on the road. For whatever reason. Lets make it fun.

There's a reason why signs are not allowed on the sides of roads for any lengthy period of time. You'd have to let everyone do it. It would be dangerous and distracting if dozens of signs and displays were allowed on the side of the road. And there would be a bad precedent set. Just think of the free advertisement space business would flock to use. It would be an advertisement orgy.