The interesting thing about this is that it's very nearly a First Amendment infringement on the right to assemble. In order to assemble on a street–a fairly obvious place to do so, for a protest, since it has good exposure and visibility–you have to ask the police to explicitly redirect traffic on your behalf. That's not a small hurdle.
It is absolute if you aren't a weasel or have a political axe to grind (the supreme court):
"Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances."
