As the City Council prepares for another vote today on mass transit, two of Hawai'i's leading independent transportation experts are at odds over ridership projections for the city's proposed rail line.

One thinks the projections are "fatally flawed." The other believes they are "reasonably accurate, reliable and valid."

The two are members of a task force that will make a report to City Council members before they take a final vote this month to choose a locally preferred alternative for mass transit. If and how the two resolve their differences could prove a key factor in the council decision, officials said.

In its Alternatives Analysis Report on transit now before the council, the city administration projects that a proposed rail line would result in 62,000 new transit riders by the year 2030.

Based in part on that, the city has recommended that the council approve building a rail line between Kapolei and urban Honolulu, rather than develop several other alternatives, including managed traffic lanes and improved bus service.

Karl Kim, a University of Hawai'i professor of urban and regional planning who has written extensively on transportation issues, said he reviewed the city's alternative analysis and concluded "the ridership forecasts were done using sound methods, providing useful information that is reasonably accurate, reliable and valid."

"If anything, the ridership estimates appear to be somewhat conservative," he said.

Another expert at UH disagreed. "I believe the report is anything but reasonable," said Panos Prevedouros, an engineering professor who specializes in transportation issues.

He accused city officials of manipulating data to ensure that the managed lane alternative "has been engineered to fail in comparison to rail."

Prevedouros said the city's past and current transit ridership figures are 21 percent too high. Using that figure to project future ridership compromises future projections, he said. He faulted the city for using "unrealistic projections" of population growth in 'Ewa Beach, Kapolei, Ko Olina and Kalaeloa that make the rail option unsupportable.

The two, who make up a task force subcommittee, said they would meet this week to try to work out their differences.

At today's City Council meeting, members are scheduled to take up the proposed transit system for the second of two required readings. The bill then is expected to go before the Transportation and Planning Committee for a hearing on Dec. 14 and return to the full City Council for a third and final vote during the week of Dec. 21.

At this point, the bill calls for the council to pick rail over the other alternatives and includes five possible routes, which will be narrowed to one by the time of the final vote.

Kazu Hayashida, a former state Transportation Department director who heads the task force appointed by the Council's Transportation Committee to review the city's proposal, said he hopes to have the group's final report ready for review by the time the Transportation Committee takes up the bill next week.

Reach Mike Leidemann at mleidemann@honoluluadvertiser.com.