From Sunday Streets:

Sunday Streets are events that encourage recreation, community activities and fun in San Francisco. Sunday Streets closes stretches of city streets to automobile traffic, and opens them to people for several hours on a various Sundays throughout the year, so participants can enjoy a large, temporary, public space where they can bike, walk, run, dance, do yoga, or do any other physical activity. Non-profit and health organizations offer free activities and share information about their services during the event.