The San Francisco Bay Trail is a bicycle and pedestrian trail that will eventually allow continuous travel around the shoreline of San Francisco Bay. As of 2014, approximately 335 miles (539 km) of trail have been completed. When finished, the Bay Trail will extend over 500 miles (805 km) to link the shoreline of nine counties, passing through 47 cities and crossing seven toll bridges. It is a project of the Association of Bay Area Governments (ABAG).
More info: http://en.wikipedia.org/wiki/San_Francisco_Bay_Trail