The First Amendment does not protect me if I yell “Fire!” in a crowded movie theater. Why not? That kind of free speech endangers the public safety. People in the theater, hearing “Fire!” would run, perhaps injuring themselves or others in their rush to get out.
The First Amendment does not protect me if I held a demonstration in the middle of a busy intersection during rush hour. Why not? We do have a right to peaceably assemble. But not when it endangers public safety. Pedestrians holding signs in the street with cars hurtling by is a danger to their safety and that of others.