Sound Transit says a March report overestimated the cost of a possible bus-only, high-capacity transit system for the Eastside by about $1 billion because the authors misunderstood some key information from the state Department of Transportation.

The mistake accounted for half of a whopping $2 billion drop in the cost estimate for an Eastside "bus rapid transit" (BRT) system that Sound Transit released yesterday.

Critics have charged that the March report, on future transit options, was biased to favor light rail over BRT, a bus network that mimics many features of rail.

But Sound Transit officials said the error was unintentional.

"A mistake's a mistake," agency spokesman Geoff Patrick said yesterday. "We're owning up to it. ... We're very sensitive that we're anything less than transparent in putting these numbers out."

Sound Transit is beginning to piece together another package of projects to submit to urban King, Snohomish and Pierce county voters, perhaps in November 2006. Rail-versus-BRT on the Eastside is shaping up to be a major battle.

The March report was an important milestone in Sound Transit's planning. It estimated the capital cost of an Eastside BRT system, using freeway high-occupancy-vehicle (HOV) lanes, at $4.4 billion to $5.8 billion in today's dollars, compared with $4.6 billion to $6.2 billion for a light-rail system.

Rail critics smelled a rat.

Richard Harkness of the anti-rail Citizens for Effective Transportation Alternatives accused Sound Transit of "systematic bias and misrepresentation."

Yesterday, Sound Transit released a much-lower estimate for BRT: $2.5 billion to $3.5 billion. Agency planning and policy officer Paul Matsuoka said part of the discrepancy — up to $1 billion — was discovered when planners realized they had made a mistake in March.

When the state Department of Transportation (DOT) provided Sound Transit with cost figures for freeway improvements needed for BRT, Sound Transit assumed the numbers were in 2004 dollars. But because Sound Transit was preparing cost estimates in 2005 dollars, planners increased the DOT's figures to account for inflation.

But in fact, the DOT numbers should have been reduced: They were in "year of expenditure" dollars, meaning the figures assumed the projects would be built in some future year. The DOT had already factored in inflation, and well beyond 2005.

Sound Transit and the consultants who prepared the March report share responsibility for the mistake, Matsuoka said. "It was something that neither one of us caught," he said.

Matsuoka said BRT's estimated price tag dropped another $1 billion when planners, at the suggestion of outside experts, reduced the estimated number of ramps that would be built to link HOV lanes on Interstate 405 with HOV lanes on Interstate 90 and Highway 520.

Those ramps would allow BRT buses to move from one freeway to another without weaving through general traffic.

The March report assumed they would be built in all four quadrants of the I-405-to-I-90 and the I-405-to-I-520 interchanges.

Later, however, members of Sound Transit's outside Expert Review Panel pointed out that the planned BRT system wouldn't use most of those ramps. Buses would run between Seattle and Bellevue across I-90, with three spurs extending north to Totem Lake, northeast to Redmond and east to Issaquah.

So the revised analysis assumes new ramps in just one quadrant of each interchange: the northwest quadrant of the junction of I-405 and I-90, and the southeast quadrant of the junction of I-405 and Highway 520.

Harkness had cited Sound Transit's earlier decision to include more ramps as a sign of anti-BRT bias. He could not be reached for comment yesterday.

Matsuoka denied Sound Transit was tilting the deck in favor of rail, calling BRT a "major part of our system."

While the new estimates indicate BRT would be less expensive to build than light rail, Sound Transit's analysis still indicates a rail system would carry more riders across I-90 than buses, and the trip would take less time.

Matsuoka also told the Sound Transit board that BRT's success depends on the state Department of Transportation managing HOV lanes to keep them free-flowing. But that's a risky bet.

"With this particular mode [BRT], we cannot promise an absolute speed," said Metropolitan King County Councilwoman Julia Patterson, D-SeaTac, who sits on the Sound Transit board. "With light rail, that predictability is there."

Sound Transit planners also yesterday unveiled a possible hybrid of rail and BRT on the Eastside, with rail across I-90 from Seattle to downtown Bellevue and then on to Overlake and perhaps downtown Redmond. BRT would link Bellevue with Totem Lake and Issaquah.

Matsuoka said the capital cost, in today's dollars, could range from $2.3 billion to $4.4 billion.

The big variables: whether the tracks between I-90 and downtown Bellevue would be elevated or in a tunnel and whether trains would stop at Overlake or continue to downtown Redmond.

Sound Transit has scheduled 11 public meetings between May 23 and June 9 to discuss proposed revisions to its long-range plan, which is the first step toward crafting a package to submit to voters.

Eric Pryne: 206-464-2231 or epryne@seattletimes.com