San Francisco Critical Mass
Briefly

Critical Mass is a mass bicycle ride that takes place on the last Friday of each month in San Francisco, with no one officially leading the event, starting at 5:30 pm at Justin Herman Plaza, and extending to cities worldwide.
Critical Mass can lead to significant traffic congestion and annoyance to motorists, especially in cities like San Francisco where the event can become massive, making it advisable to avoid downtown driving on the last Friday of the month.
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