SAN FRANCISCO (CBS SF/AP) – Downtown San Francisco’s urban center of two-wheeled activism is poised to commemorate two decades of the gathering known as “Critical Mass,” a raucous and leaderless bicycle ride and street party.

San Francisco police spokesman Albie Esparza recommends that people avoid driving in downtown San Francisco Friday as hundreds – perhaps thousands – of cyclists meet in Justin Herman Plaza at 6pm to take over city streets.

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The monthly event inspired similar rides to assert cyclists’ rights in other cities across the globe. In tandem with other efforts, it also helped spur a suite of city policies that encourage bicycle commuting, including more bike lanes and bicycle parking spots in downtown buildings.

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KCBS, CBS 5 and Chronicle Insider Phil Matier

Many more residents now cycle for their transportation than did 20 years ago. The San Francisco Municipal Transportation Agency says ridership grew 71 percent from 2006 to 2011.

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