Two months after Mayor Gavin Newsom announced plans for a bike-sharing program in San Francisco, city officials are working to implement the pilot program, which will include 50 bicycles available at five stations around downtown.

Immediately following the news release — sent after Newsom, on a sister-city trip to Paris, saw that city's successful bike-sharing network — the San Francisco Bicycle Commission and other bicycling advocacy groups decried the small scale of the initial launch. The Parisian Velib system, advocates noted, began with thousands of bicycles.

"We want to expand bike sharing as much as possible as quickly as possible given the legal constraints that we're under," said Judson True of the San Francisco Municipal Transportation Agency. "That (size of program) is what the different private entities involved felt that they could manage initially."

Officials say that getting the program off the ground involves several logistical issues. The program would be funded by advertising — Clear Channel has expressed interest — and membership fees, meaning that private interests must be taken into account, as well as the actual demand for shared bicycles. Additionally, officials have limited space to build bicycle storage stations.

The city is also constrained by a 2006 court injunction that prohibits any further bicycle development on city property until an environmental review has been completed. For the time being, the five storage and transfer stations will be situated on private land.

"Everybody wants things to move faster," True said. "We've all been frustrated by the injunction over the past few years."

Bicycle sharing has been popular in several European cities, prompting U.S. cities to start programs of their own. The Washington, D.C., bicycle share network was launched last year with 120 bikes. Officials there soon found that the demand was greater than they had anticipated.

Livable Streets, an urban improvement organization, has suggested that a successful San Francisco program would need 5,328 bicycles.

San Francisco is a nationally recognized bicycle-friendly city, but it's certainly not the only one in the Bay Area. Smaller, nongovernmental organizations have had some success with similar bicycle-sharing programs in other areas, specifically the East Bay.

According to Dave Campbell, head of the East Bay Bicycle Commission, 10 years ago the city of Berkeley conducted a study and found that perhaps Berkeley's terrain might make it too easy to steal bicycles.

"The main concern was that bike sharing works in contained areas, such as San Francisco, which has water on three sides," Campbell said. "(Elsewhere) there might be too many places to take a bike."