State rail officials are defending as reasonable their estimates that passengers will take at least 29 million trips a year on high-speed trains between San Francisco and Los Angeles by 2040.

In car-loving California, state High-Speed Rail Authority board members say, it will take only a small share of the millions of trips now being made by car and airplane to switch to trains to make the project profitable.

"If you look at the long-term projections for rides in this state," said Michael Rossi, an authority board member and former vice president of Bank of America, "we only need less than 3% total ride changes [to high-speed trains] from cars and aviation to break even, and more of that will come from cars than aviation."

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The authority's board members are going on the offensive in hopes of turning public opinion back in favor of their bullet-train plans. Critics have railed against the business plan for the project, which calls for a 520-mile Phase 1 connecting the Bay Area and Los Angeles/Anaheim, and ultimately an 800-mile system when lines are extended to Sacramento and San Diego.

New ridership estimates released in a Nov. 1 business plan are critical to making the case that high-speed trains can attract enough passengers to cover operating costs. Proposition 1A, a $9 billion high-speed rail bond measure approved by California voters in 2008, requires that the train system operate without public subsidies after the tracks and infrastructure are built.

Rossi and board chairman Dan Richard said the authority has tried to aim low with its new projections, using the least advantageous combinations of factors to plug into a complex formula.

"If you look at the inputs to the model, it's pretty hard to find one that's biased to the high side," Richard said.

"We used gasoline prices that are $1.20 a gallon below what Caltrans uses for planning. We used population numbers that are below the lowest Department of Finance projections. We used lower vehicle-efficiency levels. We make the worst assumptions on the proportion of business travel, which is what uses most of the high-speed rail."

The result produces low-end estimates of 2,700 boardings each day in Fresno, 3,200 in Merced and 2,800 in Bakersfield when the rail system reaches all the way to its proposed ends in San Francisco and Los Angeles/Anaheim.

The question remains: Where would all those passengers come from?

Among all modes of transportation -- automobiles, airplanes, buses and trains -- about 250,000 trips are made daily -- more than 91 million per year -- between San Francisco and Los Angeles, the authority reported. That number is expected to grow as California's population grows.

San Francisco International Airport spokesman Michael McCarron said there are more than 500 flights a week between San Francisco and airports in the Los Angeles Basin, including Los Angeles International, Burbank, Ontario and Santa Ana.

Three of the five busiest Amtrak routes in the nation are the Pacific Surfliner -- between San Diego and San Luis Obispo -- the Capitol Corridor -- connecting Sacramento to San Jose and Oakland -- and the San Joaquin route through the Central Valley. The three services carried more than 5.4 million riders in 2011, a record for the combined services.

Some of the harshest criticism to the rail authority's estimates has come from Valley lawmakers.

Republican congressmen Jeff Denham, R-Turlock, Devin Nunes, R-Visalia, and Kevin McCarthy, R-Bakersfield, in an op-ed piece in The Bee, described the authority's projections as "hope and fanciful estimations, not facts."