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.
San Francisco Critical Mass, April 29, 2005.
Broadway Tunnel (San Francisco) September 29, 2006
Critical Mass riders in Atlanta
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Safety in numbers is the hypothesis that, by being part of a large physical group or mass, an individual is less likely to be the victim of a mishap, accident, attack, or other bad event. Some related theories also argue that mass behaviour can reduce accident risks, such as in traffic safety – in this case, the safety effect creates an actual reduction of danger, rather than just a redistribution over a larger group.
Critical Mass, San Francisco, April 29, 2005 and Muni Metro tram on J Church line
Amsterdam, 1982