Perhaps you've seen it snaking through your city on the last Friday of every month: a street-wide armada of cyclists pedaling east to west, north to south, over hills and through the city center. Who are all these folks taking over the streets in more than 300 cities worldwide?

Click on all photos to embiggen

They're participants in a long-revered celebration of bicycle community, defiance, pride and activism known as Critical Mass.

September 28th marks the 20th anniversary of Critical Mass, and while the movement has riders worldwide, San Francisco is its birthplace.

What Is Critical Mass?

Critical Mass is an "organized coincidence" of human powered vehicles. There are no leaders or fees, there isn't even an organization. Cyclists gather once a month and take to the streets, the ride being the only constant. Some come for the camaraderie, some for the enjoyment and many to support the bicycle as legitimate traffic. Others ride as an act of defiance against a culture that values petroleum and isolation over sustainability and community.

Or, it means none of that. Attend any CM and you'll find as many reasons to ride as there are riders present. 

Such a congregation of riders is bound to cause drivers a headache, as some cars, unaware of the monthly event, find themselves literally engulfed. And while the riders are known for shepherding stuck cars off to side streets and handing out treats to stalled drivers, Critical Mass is no stranger to controversy.

While the rides celebratory spirit has been allowed to flourish in San Francisco, Chicago and more, New York City "massers" have been unjustly harassed by the NYPD in attempts to quell the ride. Veteran massers worldwide provide support on the road, and advice online, on how best to deal with aggressive drivers, cops and trouble scenarios. Each cities expression of CM is a unique experience.

Want To Get Involved?

Click here for an extensive, but unfortunately incomplete, list of ride times all over the world. Don't see your community listed? Start your own ride and help spread the movement.

For SF Locals

Several massers have organized events during the week of September 24th-30th, including theme rides, movies, open mics, symposiums, a concert, and what will likely be the largest Critical Mass ride in San Francisco history on Friday, 9/28.

For details and to join click here. To own a piece of history, check out local artist and Critical Mass instigator Hugh D'Andrade's poster for the celebration. Part of the proceeds support the 20th Anniversary Welcome Committee.

Click here to buy the 20th anniversary print

Mission Bicycle Company will be closing early, at 5pm on Friday 9/28, so all employees can attend the ride. Happy birthday Critical Mass, see you on the streets.