Sure but let's be fair the enforcement is really easy if the cops actually do their job.

See a bunch of cunts with signs standing around in the crosswalk. "Hey get out of the street or we'll arrest you." And if they don't move, arrest them.

Countries with functional police forces don't have the problem with these kinds of protestors. Guy wants to wave a sign and take a megaphone screaming bloody murder at random passerbys, more power to him. He can be safely ignored. Annoying but that's the price of a free society.