Cyclists in the Castro at San Francisco Critical Mass - Stock image
Crowd, Protest, Bicycle, California, Castro District
San Francisco, California - June 26, 2009: Critical Mass is a bicycling event typically held on the last Friday of every month in over 300 cities around the world. The ride was originally founded in 1992 in San Francisco. The purpose of Critical Mass is not usually formalized beyond the direct action of meeting at a set location and time and traveling as a group through city or town streets on bikes, attempting to halt all normal city traffic.