The Southern Crossing is a proposed highway bridge that would span San Francisco Bay in California, somewhere south of the San Francisco–Oakland Bay Bridge and north of the San Mateo–Hayward Bridge. Several proposals have been made for over the past half-century, varying in design and specific location, but none of them have ever been approved due to cost, environmental and other concerns.
The idea for the Southern Crossing dates back to the 1940s when several additional bridges across San Francisco Bay were studied. One early proposal called for the bridge to span between Third and Army Streets, with a direct connection to the Bayshore Freeway, in San Francisco to near Bay Farm Island in Alameda. Another idea was to connect the bridge directly to Alameda, enabling eastbound traffic to travel across that city and through the Posey and Webster Street Tubes to a point in Oakland near the present-day western terminus of Interstate 980. Yet another alternative was to include both the Alameda and Bay Farm Island connections, and build a junction in the middle of the bay off the western shore of Alameda. In addition, architect Frank Lloyd Wright and engineer Jaroslav Joseph Polivka collaborated on designing a reinforced concrete "Butterfly Bridge", consisting of twin 2000-foot long arches providing 200 feet of clearance above the bay's main ship channel. However, such proposals never got beyond the drawing board because of cost concerns. A bond measure was put in the ballot in 1972 that would have funded a planned crossing from Hunters Point and Alameda, but voters rejected it. Then in the 1980s, State Senator Quentin L. Kopp attempted to pass legislation to help build the Southern Crossing, but that also died after facing opposition from environmental groups.
U.S. Senator Dianne Feinstein revived the Southern Crossing proposal in 2000, writing a letter to California Governor Gray Davis that a study of an alternative bay crossing "must be undertaken promptly", citing projections on growing traffic congestion. But after a two-year study, the Metropolitan Transportation Commission concluded that it would cost at least US$8.2 billion to build a bridge from Interstate 238 in Hayward to Interstate 380 in San Bruno. The idea was then shelved until 2010 when the Bay Area Toll Authority voted to spend up to US$400,000 to conduct another two-year study on the proposed bridge.