Critical Mass is a bike ride typically held on the last Friday of every month in cities around the world where bicyclists and, less frequently, skateboarders, roller bladers, roller skaters and other self-propelled commuters take to the streets en masse. Critical Mass is not led, and has no officially-stated message, though it is largely understood to be an effort to promote alternative (non-motorized) modes of transportation and to raise awareness about safety issues commuters using non-motorized forms of transportation face in sharing the streets with motor vehicles. Participants meet at a set location and time and enjoy the security and companionship of travelling as a group through city streets intentionally causing a mild and temporary disruption to vehicle traffic. The general philosophy behind the ride is expressed in the widely-used slogan,
"We aren't blocking traffic; we are traffic."
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Source:Wikipedia Critical Mass
Image: Via Vancouver CM