Critical Mass

Critical Mass is an event typically held on the last Friday of every month in cities around the world where bicyclists and other self-propelled commuters take to the streets en masse. While the ride was originally founded with the idea of drawing attention to how unfriendly the city was to bicyclists,[1] the leaderless structure of Critical Mass makes it impossible to assign it any one specific goal. In fact, the purpose of Critical Mass is not formalized beyond the direct action of meeting at a set location and time and traveling as a group through city or town streets.

in http://en.wikipedia.org/wiki/Critical_Mass