Blocking traffic is not legal and is not a new practice for protesters. When protesters block traffic, they are engaging in civil disobedience, a term coined by one of America's earliest freethinkers and intellectuals, Henry David Thoreau.
While nearly everyone caught in a traffic jam caused by protesters becomes upset due to the delay, it is important to recognize that reporting on traffic conditions is a mainstay of local news stations across the country, while protests often get ignored. Blocking traffic means at very least making the local traffic report.
Although organized protests or marches can obtain permits to close streets, frequently protesters move from the permitted areas. When protesters block highways or streets that they are not permitted to be on, they do risk arrest. However, police are loathe to arrest peaceful protesters, even when they block traffic. The recent protest in Washington D.C. blocked a busy intersection for 7 minutes, and there were no arrests reported.