Editor's note: The slideshow above provides a look at where bridge proposals from the last 70 years would have crossed the San Francisco Bay.

San Francisco structural engineer Roumen V. Mladjov noted last year in a Chronicle opinion piece that there are 700,000 people per bridge in the Bay Area compared to only 171,000 for New York City.

The demand on the Bay Bridge alone results in gridlock even on weekends and weekday rush hour starting at 5 a.m., all while the region's population is expected to increase from the current 7.7 million to 9.3 million in 2040.

Sen. Dianne Feinstein has said for years that the bay needs a new bridge. BART is just as adamant that the time has come for a second Transbay tube. By the time Feinstein made a push in 2000, calls for a companion to the Bay Bridge had been bubbling up for more than 50 years.

In fact, the Southern Crossing, a highway bridge that would span the bay somewhere between the Bay Bridge and the San Mateo Bridge is an idea brought forward more than a dozen times over the last 70 years, only to founder. Below is a look at each major proposal and what went wrong:

1946-49 — The Army-Navy Bridge

Proposal: A joint Army-Navy board of engineers proposes immediate construction of low-level, six-lane combination trestle and tube.

Terminuses: Western — the foot of Army Street in San Francisco. Eastern — Fifth Street in Alameda, just south of the Alameda Naval Air Station.

What happened: The Chronicle wrote at the time that a new bridge was "assured." It wasn't. Cost concerns and political disputes scuttled the project. However, the general location of the proposed span would continue to be considered in future bridge projects.

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1946-48 — The Butterfly Bridge

Proposal: After the construction of the Bay Bridge, the great architect Frank Lloyd Wright and renowned engineer J.J. Polivka envision a new bridge to the south — a 200-foot-high reinforced concrete, six-lane "Butterfly Bridge," the main section consisting of twin arches flung across 2,000 feet of water and resting on huge hollow piers. Motorists could pull off to a rest area with hanging gardens between the arches. Fun fact: A model of the bridge was used as a prop in the 1988 action thriller "Die Hard."

Terminuses: Western — Army and Third streets in San Francisco. Eastern — Bay Farm Island north of Oakland Airport.

What happened: Never went further than the design stage.

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1969-1971 — The Foley Bridge

Proposal: Gov. Ronald Reagan and Bridge Authority Chief Engineer E.R. "Mike" Foley revive the plans for a new bridge. Reagan actually casts the deciding vote on the bridge design, opting for a plan with twin towers instead of one employing a 1,300-foot-high arch.

Terminuses: Similar to the Army-Navy plan. Western — India Basin in San Francisco. Eastern — somewhere in Alameda.

What happened: Opposition from multiple fronts — political agencies in SF, Alameda and San Leandro, the San Francisco supervisors and Assemblyman Leo Ryan — helped quash the project until a study assessing the impact of the new BART system on Bay Area traffic could be undertaken. The fear was that a second bridge would siphon commuters away from the brand new light rail system. We're still waiting on that study.

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1971 — Southern Crossing Bond Initiative

Proposal: A new bridge with a similar route, but this time funded by a bond measure.

Terminuses: Western — Hunters Point in San Francisco. Eastern — freeway splits with a northern route connecting to Alameda's Webster Street and the southern leg with Bay Farm Island.

What happened: Voters soundly defeat the measure by a margin of 3 to 1 in 1972.

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Late 1980s — The Kopp Crossing

Proposal: Then state Sen. Quentin Kopp proposes a 10-mile bridge across the Bay north of the San Mateo Bridge.

Terminuses: Western — South San Francisco, presumably hooking up with Highway 101. Eastern — Hayward.

What happened: San Mateo County leaders and environmental groups successfully fight the plan.

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2000, 2012 and 2017 — DiFi's Southern Crossing

Proposal: In a letter to then Gov. Gray Davis, Sen. Dianne Feinstein calls for a new bridge to relieve the Bay Area's growing traffic congestion. A two-year study puts the cost at $8.2 billion.

Terminuses: Western — I-380 in San Bruno. Eastern — Highway 238 in San Leandro.

What happened: The estimated cost, Sierra Club and other environmental group opposition, and the priority given to refitting the existing Bay Bridge combine to deliver a death blow to the 2000 proposal.

Update 2012: A second study of the Hwy. 238-Peninsula bridge, this one carrying both cars and some form of transit, is completed. The cost now has jumped to $12.4 billion.

Update 2017: Feinstein renews her call for a Southern Crossing span, telling the Metropolitan Transportation Commission in a letter, "The traffic demands on our streets and transit systems have become intolerable." She says such a span could carry highway traffic, light rail, BART and/or autonomous vehicles.

MORE: These are the craziest secrets in the history of BART

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2014 to present — Second Transbay Tube

Proposal: A second Transbay Tube to relieve BART overcrowding and serve a growing East Bay population. (BART is currently expected to reach maximum capacity in 2025.) Whether it should include standard gauge rails for non-BART trains, like Caltrain, has not been decided. Cost is estimated at $12 billion to $15 billion.

Terminuses: Western — link up with BART lines in San Francisco, anywhere from Montgomery Street to Mission Bay. Eastern — somewhere in Alameda.

What happened: Still in planning stages. BART plans to hire a consultant this month to discuss issues concerning the construction.

For more information about the proposed Transbay Tube, see the San Francisco Chronicle's transportation guide: The Bay Area's 11 biggest transportation projects.

RELATED: SF peak hour commuters lost nearly 5 days to traffic congestion — study

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Read Mike Moffitt's latest stories and send him news tips at moffitt@sfgate.com.

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