For just a couple of hours on the last Friday of each month, hundreds of cyclists informally ride together to claim the streets which are normally ruled by automobiles. The gathering is part celebration and part peaceful protest. The main message: we still have a choice to live our lives in a way that is more thoughtful of the world's resources, to live in a way that is more socially nurturing of life and health, and to live our lives with more joy. The ride on September 28th marked the spontaneous demonstration's 15th year.
Photos | Wikipedia: Critical Mass | Tribe.net: SF Critical Mass

Previous Indybay Critical Mass Coverage:
2007: San Francisco Bike ride against G8 | May Berkeley Critical Mass Ride Ends With Three Bikes Trapped Under Car | Oakland Critical Mass Riders Repair Bikeway, Storm Big Boxes | Walnut Creek Critical Mass
2006: Bicycle Film Festival in SF | Santa Cruz: Mad Max on Bikes | Santa Cruz Critical Mass Bike Ride
2005: Critical Mass is Thirteen | Sacramento Critical Mass Meets Brutal Force
2004: Halloween Critical Mass Tours Locked-Out Hotels
2003: Critical Mass Celebrates its 11th Year | Bikes Not Bombs and Critical Mass Take To The Streets
2002: Happy 10th: Thousands of Cyclists Reach Critical Mass in SF | 10 Year Critical Mass Bike Ride | Bicycle activists Sue Sacramento Police
2001: Local Bicycle Activists Jam the Streets | Police Abuses at Sacramento Critical Mass