Critical Mass is a form of direct action in which people travel as a group on bicycles at a set location and time. The idea is for people to group together to make it safe for each other to ride bicycles through their streets, based on the old adage: there's safety in numbers.

Critical Mass events highlight the numbers of people who want to use their bicycle on the streets, but are usually unable to do so without risking their safety. They are a call to action to councils, governments and road planners to properly and thoughtfully design in the safety of all road users, including those who would prefer to walk and cycle, instead of prioritising motor traffic above all else.

The event originated in 1992 in San Francisco (typically held on the last Friday of every month);^[1] by the end of 2003, the event was being held in over 300 cities around the world.^[2]

Critical Mass has been described as "monthly political-protest rides", and characterized as being part of a social movement.^[3] It has been described as a "monthly protest by cyclists reclaiming the streets."^[4] Participants have insisted that these events should be viewed as "celebrations" and spontaneous gatherings, and not as protests or organized demonstrations.^[5][6] This stance allows Critical Mass to argue a legal position that its events can occur without advance notification of local police.^[7][8] Though they are sometimes described as political, they are not necessarily.

Critical Mass-like bicycle tours with hundreds of participants took place in Stockholm, Sweden in the early 1970s.^[9] But the first ride within the present wave took place on Friday, September 25, 1992, at 6 pm in San Francisco. At that time, the event was known as Commute Clot and was composed of a couple of dozen cyclists who had received flyers on Market Street.^[1]

Shortly after this, some participants in that ride went to a local bicycle shop for a screening of Ted White's documentary Return of the Scorcher, about bicycle culture in the Netherlands and China in comparison with that of the United States. In that film, American human-powered vehicle and pedicab designer George Bliss noted that, in China, both motorists and bicyclists had an "understood" method of negotiating intersections without signals. Traffic would queue up at these intersections until the backlog reached a "critical mass", at which point that mass would move through the intersection. This term from the footage of the movie, was applied to the name of the ride,^[10] and the name caught on, replacing "Commute Clot" by the time of the second event.^[1]

Critical Mass has a decentralized (rather than hierarchical) structure. Critical Mass is sometimes called an "organized coincidence", with no leadership or membership. The routes of some rides are decided spontaneously by whoever is currently at the front of the ride, while others are decided prior to the ride by a popular vote of suggested routes often drawn up on photocopied fliers. The term xerocracy was coined to describe a process by which the route for a Critical Mass can be decided: anyone who has an opinion makes their own map and distributes it to the cyclists participating in the Mass. Still other rides decide the route by consensus. The disorganized nature of the event allows it to largely escape clampdown by authorities who may view the rides as forms of parades or organized protest. Additionally, the movement is free from the structural costs associated with a centralized, hierarchical organization. In order for the event to function, the only requirement is a sufficient turn-out to create a "critical mass" of riders dense enough to occupy a piece of road to the exclusion of drivers of motorized vehicles. Authorities in New York, California and Oregon have expressed concern with the difficulty of coordinating with the riders, due to the lack of leadership.^[11][12][13]

Critical Mass rides vary greatly in many respects, including frequency and number of participants. For example, many small cities have monthly Critical Mass rides with fewer than twenty riders which offer safety in numbers^[14] to cyclists in those locales, while on the opposite extreme, in what have been the largest events using the name Critical Mass, cyclists in Budapest, Hungary hold only two rides each year on April 22 (Earth Day) and September 22 (International Car Free Day). The "Budapest style" attracts tens of thousands of riders.^[15] The April 20, 2008, Budapest ride participation was estimated at 80,000 riders.^[16][17] In Vienna, close to Budapest, a Critical Mass Ride has been held every month since 2006 and attracts up to 1,000 or more riders.^[18]

In the Critical Mass practice of "corking", a rider breaks away from the group to block the side streets of an intersection as the mass crosses. This prevents traffic travelling through the intersection on a green signal and allows the riders to ride through red lights. This both contains cross-traffic while the mass passes and protects the mass from splitting or from drivers who might attempt to pass through the mass.^[19]

Critical Mass rides have generated controversy and public opposition.^[47] The group has often protested in high-profile events, ranging from major political events^[48] to the Olympics.^[49] Some critics claim that Critical Mass is a deliberate attempt to obstruct traffic and disrupt normal city functions, asserting that individuals taking part refuse to obey traffic laws.^[50]

Some bicycling advocacy groups have expressed concern that the nature of Critical Mass and altercations with motorists could weaken public support for cyclists.^[12] Though it does not condone incidents of violence and rudeness, the San Francisco Bicycle Coalition credits Critical Mass with spotlighting bicycle issues and aiding their efforts in advocating for cyclists.^[1]