The high-speed rail rider estimates the state used to sell voters a $10 billion bond and a massive overhaul of the Caltrain line are flawed and cannot be trusted, a leading group of transportation experts said Thursday.

A UC Berkeley Institute of Transportation Studies report says the data used by bullet-train planners is so "unreliable" that it is impossible to predict whether the project will be successful or lead to "severe revenue shortfalls."

The study also questions the state's choice to zip the trains up the Pacheco Pass to the Caltrain tracks instead of through the East Bay because the Peninsula route would purportedly attract more riders. That decision — which is likely to lead to heavy property-seizing along the line from San Francisco to San Jose — should be re-examined using better data, the authors said.

Rail officials stood behind their figures and said the Berkeley report was rooted in academia and out of touch with the real world.

The professors said the firm that devised the estimates for the California High-Speed Rail Authority unfairly changed its formulas to show higher rider counts after the initial results did not meet the authority's expectations. The firm also surveyed a disproportionate number of likely high-speed rail users instead of a random sample of the public.

"This is quite an indictment. Nobody was watching the store," said state Sen. Alan Lowenthal, D-Long Beach, chairman of the state Senate Transportation and Housing Committee, which called for the report and has been overseeing the high-speed rail project. "It makes you think after reading this that they told (their consultants) that they wanted certain things to come out in a certain way."

A lot is riding on the ridership estimates being accurate.

In addition to being a main justification for spending $43 billion on the state's largest project, the ridership projections are tied to claims that the system will pay for itself once built — with no taxpayer subsidies — and fund extensions to Sacramento and San Diego. It is also a key selling point as the authority tries to attract $10 billion to $12 billion in private investments.

The rail authority, which funded the study, and Cambridge Systematics, a Massachusetts-based firm that developed the rider forecasts during the past three years, took issue with nearly all the conclusions made in the report.

"The authority remains confident that the ridership model we've been using has been and will continue to be a sound tool in planning the high-speed rail system," rail authority Deputy Director Jeff Barker said.

The project is scheduled to break ground in fall 2012 and start zooming passengers along the Caltrain tracks on their way between San Francisco and Los Angeles at speeds of 125-220 mph by 2020.

When voters approved the project via a $9.95 billion high-speed rail bond in 2008, the state said the system would eventually attract 55 million annual riders. The federal government awarded the project $2.25 billion in stimulus money in January after the state sliced its estimate down to 41 million riders because it nearly doubled projected fare prices.

The study comes after highly critical reports of the high-speed rail project by the Legislative Analyst's Office and state auditor released separately this year.

Cambridge CEO Lance Neumann, in responding to the report, said they "stand firmly behind" their work, citing nearly 40 years of experience.

"We emphatically disagree with the author's conclusions that the model is not reliable," Neumann said.