SAN FRANCISCO -- It's a dream that refuses to die, even though no one knows where to get $550 million to build a bike and pedestrian lane on the west span of the Bay Bridge.

Cycling advocates consider it their top goal to have a biking and walking lane all the way across the bridge from Oakland to San Francisco to rival that of the Golden Gate Bridge.

And they're nearly halfway there. The new east span of the Bay Bridge from Oakland to Yerba Buena Island is scheduled to open in late 2013 with a bike lane.

Bridge operators this week released a report with mixed news on the prospect for extending that lane to the west span to San Francisco.

Inflation has increased the estimated cost of a western bike lane to roughly $550 million -- nearly double the $300 million price tag a decade ago, the Metropolitan Transportation Commission concluded in the draft study.

"We don't have any identified sources of money for the project," said Peter Lee, MTC's senior program manager, on Tuesday night at a San Francisco meeting to release the study.

Yet completing the $1.2 million study next year will open the door for the commission and Caltrans to begin seeking funds for it, commission staffers said.

"We have a project that is technically feasible, but we haven't determined if it is financially feasible," said John Goodwin, an MTC spokesman. "This study is a first step. In that regard, the Bay Bridge was built in 1936, but the first proposal for the project was made in something like 1876."

Transportation planners say a bike lane across the entire Bay Bridge would fill a void. There currently is no bridge crossing for walkers and cyclists between the Dumbarton and Carquinez bridges -- which both have bike lanes.

Tourists, recreation users and bike commuters would use a lane connecting San Francisco, Oakland and Berkeley, said Dave Campbell, program director for the East Bay Bicycle Coalition.

"We know more people would use a bike lane across the Bay Bridge than the one on the Golden Gate, and look how popular that one is," Campbell said. "The weather is a lot better over here."

He suggested the Bay Area Toll Authority -- the MTC wearing its hat as toll bridge managers -- consider a temporary toll hike on the Bay Bridge to fund a west span bike lane.

Transportation commissioners, however, have said they are reluctant to raise tolls any time soon after an increase in July 2010 that boosted the Bay Bridge toll from $4 at all times to $6 during rush hours on weekdays and $5 on weekends.

The design for the proposed bike lanes won't be determined until further studies are done, but some design elements are emerging.

MTC engineers said Tuesday it appears impractical to build a new bike path on just one side of the bridge. The extra load and wind resistance would put uneven stress on the bridge.

As a result, the engineers recommend cantilevering new paths on both sides of the bridge. The north side would be used for bike and hiking lanes in both directions. The southern side would be used for Caltrans maintenance vehicles.

Engineers also said the extra weight of bikes lanes could cause the main bridge structure to sag and interfere with clearance of big ships passing underneath.

To mitigate the sagging concern, Caltrans could install lighter material in the bridge deck or shorten the suspender cables holding up the deck, the study says.

A bike lane will be on just one side of the new east span, but that structure was designed from the start to accommodate the weight.

Daniel Feldman, a San Francisco resident, said he would use a bike lane across the Bay Bridge sometimes to commute to his job in Berkeley.

"It's the workout to work approach," Feldman said, "and it would be very scenic. ... "I could see riding across the bridge to work, assuming I'm still young enough to do that when the lane is finally built."

Contact Denis Cuff at 925-943-8267. Follow him at Twitter.com/deniscuff.