Though the idea of a second BART tube has been discussed over the past 25 years, in just the past few months it has assumed a striking new momentum.

A front-page article in The Chronicle on a second tube early this year has been followed by several opinion pieces in favor of the idea, blog postings and declarations of support from Mayors Ed Lee of San Francisco and Libby Schaaf of Oakland. San Francisco Supervisor Scott Wiener is actively pushing the idea, as are Lee and his staff. In April, the BART board will be asked to support a major study of planning and evaluation.

So, as the momentum develops, it is time to briefly note the changing travel dynamics driving this momentum, where the funding might come from, and whether the project may finally have reached a tipping point.

Travel patterns in the Bay Area have been evolving since the opening of the BART system 43 years ago, and continue to evolve. One of the central principles on which the BART system was built, that of commuters coming from housing in the suburbs to the central cities of San Francisco and Oakland, was already out of date when BART opened in 1972. And the dispersion of jobs throughout the region and multiplicity of commute routes has only grown since.

What has changed in the past few years, though, are several dynamics of system capacity and backup. First is the heightened concerns about the fragility of a single tube: The tube is in need of repairs and has no backup if there is a breach, through intentional act or accident. Second, the intense residential and commercial development pattern of South of Market has generated demand for direct transit service to that area, as well the growing interest in infill stations in the East Bay and San Francisco. Third, and perhaps most important, is the lack of capacity. As any BART rider can attest, the system is overloaded, as demonstrated by standing-room-only during rush hours, more frequent delays and breakdowns.

Several options for a second tube have been presented that would bring major transit gains. One option is for a line from the MacArthur Station that would go south, with stations at Lakeshore/Grand and Ninth/Laney in Oakland, a station in Alameda, and then a second BART crossing to Mission Bay, with several stations in Potrero Hill and the Bayview. A second option is to cross from Alameda to Mission Bay, continuing to Civic Center and then out to the west side of San Francisco.

From the start, two central issues of cost and timing must be addressed. Major infrastructure projects almost always achieve high positive polling numbers when they are described in the abstract. Enthusiasm wanes when cost is introduced. The early, rough estimates for a second tube are in the range of $10 billion to $12 billion. Funding suggestions have revolved around using state cap-and-trade money, federal and state transportation funds, parking fees and vehicle license fees. Even if possible, these would not be sufficient. From the start, it should be clear that some new source of dedicated revenue will be needed requiring a Bay Area vote. That’s the way it should be.

Second, if business-as-usual is followed, a second tube can take two decades or more (some estimates are for 30 years). Any project approach must include a sense of urgency and willingness to expedite processes, before any implementation funding is committed. If on a small scale, the rebuild of the Santa Monica Freeway in Southern California in 1994 and the rebuild of the MacArthur Maze in 2007, both completed months before the most optimistic projections, show the possibilities when a sense of urgency is present, and expedited processes are employed. The rebuilding of the Oakland-San Francisco Bay Bridge, years behind schedule, shows the opposite.

Large-scale California transit projects nearly always have required a long period of gestation, in which concepts are presented, discussed, dismissed as impractical or too expensive, then set aside. But they may not be forgotten, and a combination of forces come together to achieve a type of tipping point in implementation. Traffic gridlock significantly worsens, or technological advances increase feasibility or reduce costs, or new funding becomes available, or usually a combination of these elements, combined with one or several project champions. This is the arc largely followed by the fourth bore of the Caldecott Tunnel, the Highway 4 widening, the BART extension to San Jose, and even for the basic BART system itself — which was first seriously discussed in the late 1940s.

It may be that the second tube is reaching its tipping point. The BART board’s decision in the next month will be one indication.

Michael Bernick is a former BART director (1988-96). To comment, submit your letter to the editor at www.sfgate.com/submissions.