The Southern Crossing is a proposed highway structure 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 since 1947, varying in design and specific location, but none of them have ever been implemented because of 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. After the Bay Bridge crossing opened in 1936, connecting Rincon Hill  2  in San Francisco with the Key Mole  5  in Oakland via two high-level bridges and a tunnel through Yerba Buena Island, vehicle traffic exceeded estimates almost immediately; by 1945, even with gasoline rationing, traffic was 191% of the estimates made during planning, and would reach an average of 69,000 vehicles per day by 1946. A second crossing was deemed necessary and studies were initiated to determine the best location and configuration.

A study completed on November 18, 1941 conducted by a joint Army-Navy Board concluded that a high-level bridge between Hunters Point  4  and Bay Farm Island  8  was feasible at an estimated cost of $53.2M. The high-level bridge was preferred to a low-level lift bridge. The western terminus would have been on the south side of the existing Hunters Point Naval Shipyard and the eastern terminus would be on the southwest shore of Bay Farm Island. However, no immediate need was identified for a new bridge, and the project was shelved.

 I personally believe that both crossings are needed and should be constructed as early as possible. I believe our plans should be projected on that premise. If it were financially possible to do so I would vote for their construction simultaneously. But it cannot be done. Any new crossing must be financed by revenue bonds and the experts who pass upon such matters advise that we could not sell the bonds of two crossings at one time.

On January 25, 1947, another joint Army-Navy Board published a study which concluded that a combination of causeway and tube from San Francisco (near Army Street/Potrero Point  3 ) to Alameda  7  was preferred. From Alameda, the route would run north through the Posey Street tube to a point in Oakland near the present-day western terminus of Interstate 980. This was essentially the same as alternative no. 12 from the contemporaneous California Department of Public Works (DPW) study. Both routes recommended by the two 1947 studies were endorsed by Charles H. Purcell, the director of the California DPW on November 10, 1947; however, because of limited budget, the more expensive Southern Crossing option would be built after the Parallel Bridge.

During the development of the Joint Army-Navy Board study, a public meeting was held on August 13, 1946, where ten alternative alignments for a second crossing were suggested. One group, based in San Francisco, advocated for a low-level causeway allowing the extension of the transcontinental rail line from Oakland; another group, based in the East Bay, proposed a high-level span for automobiles only. The California state government, represented by engineer F. W. Panhorst, agreed with the East Bay group and further advocated that to provide the maximum reduction in traffic on the Bay Bridge, the western terminus should be on Telegraph Hill  1  or Rincon Hill  2 . The August 1946 meeting was also notable for the first public presentation of the Reber Plan, which called for an immense dike between San Francisco and Oakland 2,000 feet (610 m) wide, reclaiming 180,000 acres (73,000 ha) of land from San Francisco Bay and eliminating the need for another bridge. The 1947 Joint Army-Navy Board considered 29 different alignments before concluding the causeway-tube was the preferred option.

The first of the California state government studies was published a week later, on January 31, 1947, which concluded that another trans-Bay crossing was feasible and recommended that it be located close to the existing Bay Bridge. This alignment was later named the Parallel Bridge.

The main reason to duplicate the existing route was to provide immediate relief to Bay Bridge traffic; the crossing further south from Potrero Point  3  to Alameda  7  was estimated to divert only 20% of existing traffic, and the southernmost proposal studied, from Hunters Point  4  to Bay Farm Island  8 , was estimated to divert only 5% of existing traffic. The alternative western terminus at Telegraph Hill  1  was rejected on the basis of existing traffic patterns and freeway connections, and the alternative eastern terminus on the S.P. or Oakland Mole  6  was rejected not only because it would require a long high-level span to avoid interference with the existing shipping channel, but also that such a span would also interfere with flight operations at Naval Air Station Alameda.