The mention of bike rallies usually summons images of Harleys, leather, and tattoos — but not so the rally held Friday in San Francisco to celebrate the 10th anniversary of Critical Mass. Somewhere between 3,000 and 5,000 riders turned out for the party, far more than normally participate in the monthly biking event to encourage alternative transportation and advocate for better biking conditions. The throng of riders occupied up to 40 city blocks, much to the dismay of those stuck in their cars. (In some cases, tensions between riders and drivers erupted into profanities and low-level violence.) The 10th anniversary of Critical Mass also marked San Francisco’s first-ever official Car-Free Day. Or make that part of a day; the city closed but one main thoroughfare, Montgomery Street, and that only for four hours. However, organizers hope the event will someday be as successful as Critical Mass, which, since its birth in San Francisco, has spread to cities throughout the U.S. and to many other countries.