[7] 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 study completed on November 18, 1941 conducted by a joint Army-Navy Board[8] concluded that a high-level bridge between Hunters Point  4  and Bay Farm Island  8  was feasible at an estimated cost of $53.2M.

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.

[11][12][9]: xii  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.

[4]: I-4  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.

[9]: xii–xiii 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.

[4]: I-3 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.

[3]: 18 The 1947 study also determined the feasibility of extending existing railroad termini for the Western Pacific Railroad, Atchison, Topeka, and Santa Fe, and Southern Pacific from Oakland to San Francisco; of the three, SP already had the option of using the Dumbarton Cut-off, and WP and AT&SF used buses (for passengers) or ferries (for freight); both a high-level bridge and a tunnel were ruled out as the required grades for the high-level bridge could not be achieved without excessively long approaches, and the tunnel would require conversion to electric locomotives.

However, a low-level bridge accommodating rail traffic was considered for the southernmost Hunters Point  4  to Bay Farm Island  8  crossing as alternative no.

In May 1949, San Francisco mayor Elmer Robinson testified before the California State Assembly in support of the Southern Crossing.

[16] Director Purcell went before Congress in July 1949 to request permission to expand the right-of-way on Yerba Buena Island to allow construction of the Parallel Bridge.

[4]: I-5 At about the same time, architect Frank Lloyd Wright and engineer Jaroslav Joseph Polivka unveiled the reinforced concrete "Butterfly Wing" bridge in May 1949 before Bay Toll Crossings.

[18] A 16 ft (4.9 m) long model of the Butterfly Wing bridge was built for the 1953 presentation;[18] it was later displayed locally in shops,[19] malls,[20] and was included in a traveling exhibition[21] before being used as a prop in the 1988 film Die Hard.

[22] Polivka had previously submitted preliminary plans to DPW for a Southern Crossing bridge in February 1947; his initial design featured an immense 3,200-foot (980 m) long concrete arch spanning the navigation channel, which was unprecedented in length, size, and material.

[21] The final design carries six lanes of traffic over twin 2,000-foot (610 m) long diverging arches providing 200 feet (61 m) of vertical clearance above the bay's main ship channel.

[13][29] The Butterfly Wing bridge design has been revived occasionally since it was first announced; in 1989, the Oakland Museum of California exhibited the bridge drawings and model,[24] and that year, Aaron Green and T. Y. Lin proposed the Wright/Polivka design should be built between San Bruno  12  (western terminus at the junction of I-380 and the Bayshore Freeway) and San Lorenzo  11  (terminating into Hwy 238), carrying eight lanes of traffic and BART tracks.

[24] In 1953, the legislature passed a bill to build the Southern Crossing[32] with the alignment spanning from San Francisco near Third and Army Streets (Potrero Point  3 )to near Bay Farm Island  8  in Alameda.

[33] In March 1956, the 1955 Supplement report was issued covering the legislative amendments made that month changing the San Francisco approaches to the Southern Crossing.

[34][35][36] The Southern Crossing design was updated again in October 1956; the 1956 Progress Report detailed further changes to the San Francisco and Bay Farm Island approaches mandated by legislative action.

[39]: II-5  In addition, the 1956 Progress Report considered the effect of the Southern Crossing on traffic over the San Mateo–Hayward Bridge (acquired by the State of California in September 1951) as well as the planned Transbay Tube.

[40]: I-1, 2  One key statute passed in the 1957 session imposed a July 1, 1958 deadline; if funding was not finalized for the Southern Crossing by then, the project would be dissolved.

The eastern approach would split into two at the toll plaza, located approximately 1 mile (1.6 km) offshore from Alameda; the northerly leg would continue to a new set of tubes parallel to the existing Posey and Webster Street tubes, while the easterly leg would continue to Davis Street in San Leandro.

[5]: 60–61 Of the two alignments, the India Basin–Alameda–Bay Farm Island option was expected to have the greatest relief for Bay Bridge traffic and was recommended in the 1966 study report.

[5]: 104–106 The Hunters Point/India Basin–Alameda–Bay Farm Island alignment was ultimately selected in April 1966[46]: 21  with a cable-stayed girder bridge over the main channel using an orthotropic deck.

[46]: 21 Opposition to the Southern Crossing was led by the Sierra Club and notable local politicians, including state senator George Moscone; Assemblymembers Willie Brown, John Burton, John Foran, and Leo McCarthy; and Supervisors Dianne Feinstein, Terry Francois, Robert E. Gonzalez, Robert H. Mendelsohn, and Ron Pelosi.

Because of its potential to siphon riders (and revenue) away from the nascent BART system, the California State Assembly ordered the Toll Bridge Authority to reconsider the Southern Crossing in 1970.

[46]: 4–5 In March 1971, Assemblymember Robert W. Crown (D-Alameda) sponsored AB 151, which would give the final decision to proceed with the construction to the legislature, taking that choice away from Bay Toll Crossings.

[52] Although it passed both houses with overwhelming majorities, AB 151 was vetoed by Governor Ronald Reagan,[53] who stated the citizens of the Bay Area should be allowed to vote for the approval of the Southern Crossing directly.

[1][6]: 42  Other alternatives that were studied in this period included a second Transbay Tube, widening the San Mateo–Hayward Bridge to eight lanes, and Dumbarton Rail Corridor service; of these, the Mid-Bay Bridge proved to be the most polarizing, the second Tube was the costliest, and Dumbarton Rail was called "one of the least expensive and most cost-effective of the transbay improvements studied.