In a bid to increase bicycling in Philadelphia, the city plans to designate one lane along two major streets - Spruce and Pine - for bikes, leaving the other lane for all vehicular traffic.

City workers will paint new lines along both streets, from river to river, officials said, with the pilot project beginning around Labor Day.

Philadelphia currently has 32 miles of multiuse trails (no cars) and 205 miles of bicycle lanes - but only four miles of dedicated lanes in Center City.

Cyclists can get to Center City easily, just not through it.

The League of American Bicyclists, an advocacy group, recently ranked Philadelphia in the fourth tier of bicycle-friendly communities - well below Boulder, Colo., Seattle, and San Francisco but on a par with New York and Albuquerque, N.M.

Cycling in Philadelphia has doubled in the last three years, according to the Bicycle Coalition of Greater Philadelphia.

The Center City stretch of the Schuylkill River Trail, which has seen the most explosive growth, is up to an average 16,000 cyclists a week. The end of Kelly Drive records more than 30,000.

But neither links west and east, the Schuylkill with the Delaware. That requires a dodge-'em trip directly through Center City.

"The hole in the doughnut that prevents these numbers from going even higher is connecting these facilities to where the jobs are," said Spencer Finch, a sustainable-development director for the Pennsylvania Environmental Council.

When officials studied the issue, they concluded that existing bike lanes along Spring Garden Street and Washington Avenue are too far away from Center City's historical attractions and jobs. And wider corridors like Market Street are already too congested.

Chestnut Street supposedly has a bike lane - but one that's shared with buses.

"The risk-averse are going to stay off Walnut, Chestnut, and Market," said Patrick Starr, regional vice president of the PEC.

The PEC's traffic statistics show that Spruce and Pine both have unused traffic capacity, suggesting that they could accommodate vehicles in a single lane.

"The folks who drive think the cyclists get in their way, and it's not safe," said Rina Cutler, deputy mayor for transportation and utilities. "Cyclists also sometimes ride as if there were no cars on the street. We're trying to figure out how to do this in a way that gets everyone to share the road in a way that's safe and convenient."

Cutler has promised there will be no loss of parking. People will still be able to pull to the side of the road - briefly - without being ticketed. And she said the timing of traffic signals would be reset to improve the flow along Pine and Spruce.

Bicycling and environmental advocates praised the initiative.

"We see this as a major plus," said Katie Edwards, project coordinator for the Clean Air Council. For the nonprofit advocacy group, every car off the streets amounts to an incremental improvement in air quality.

But some residents of the neighborhoods along the route - not to mention a few drivers - aren't so sure it will work.

John McSorley, who frequently drives through the area, let out a long whistle yesterday when he was told about the plan.

"I think you're asking for traffic trouble," said McSorley, who had just parked on Pine. "Especially during rush hour."

Even now, the simple act of unloading your groceries or your grandmother causes backups. The two streets at midday resemble a giant slalom course.

Heading east on Pine yesterday afternoon, there was a van in one of the traffic lanes at 11th Street, a U-Haul truck at Fifth, an appliance delivery vehicle just past Third.

West along Spruce was worse: a truck at 12th, a woman unloading groceries just before Broad, two trucks at 16th, a pickup at 17th, a moving van at 19th.

Traffic - cars, buses, and cyclists - snaked through.

Whether the plan succeeds "really has to do with how it's affecting quality of life," said Rosanne Loesch, president of the Society Hill Civic Association.

"Are there traffic backups? Is it ridiculously slow to get through the neighborhood? Is there confusion?" And what about the horse-drawn carriages so prevalent in Society Hill, and the patient drop-off entrance to Pennsylvania Hospital on Spruce?

That said, Loesch, whose husband bikes to work daily, endorses the concept. "Having safe biking is really a priority," she said, noting that the city's historical layout is "the beauty and also the challenge: How do you fit these modern and really good ideas into these little historic streets?"

For the city, the bike lanes are another step in implementing the Greenworks sustainability plan, unveiled last spring. Among other cycling initiatives, Greenworks called for an east-west corridor linking the Schuylkill River Trail and a similar major trail envisioned for completion along the Delaware River in the future.

Both are part of the East Coast Greenway network of trails, an Eastern Seaboard trail system from Canada to Key West - an urban version of the Appalachian Trail for hikers.

If the issue were just moving people between the two rivers, "there may be other places that are easier," said Joseph Hacker, manager of transit, bicycle, and pedestrian planning at the Delaware Valley Regional Planning Commission. But Pine and Spruce are near historical attractions.

"If people are going through our fair city, you want them to ride past stuff that is important," Hacker said.

Studies suggest that bike lanes do attract riders. In 1998, before bike lanes were installed on Spring Garden Street, for example, the bicycle coalition counted an average of 78 cyclists passing 13th Street during a 4.5-hour period. By 2009, after the lanes had been installed, the number rose to 339, an increase of 335 percent.

Alex Doty, the coalition's executive director, doesn't see how the current chaos could get any worse. "It's sort of a free-for-all out there."

He said a similar bike lane along 44th Street in West Philadelphia, where he lives, has actually had a calming effect on the traffic and the neighborhood.

"When two cars are going side-by-side toward a stop sign, they tend to go faster," Doty said. "You get taxis trying to pull out. Or someone is double-parked and the taxi is playing chicken. Where you have changes in lanes, that's where you have danger - for bicyclists for sure, but that's also where you see a lot of crashes for cars."

Rita Soto of Bella Vista, who was cycling along Pine Street yesterday to do some errands, said she would welcome a dedicated lane and would alter her route to use it.

Fairmount resident Katie Block, who had pulled her car to the curb of Pine Street to pick up a friend, said that making any Center City street one lane "is probably a bad idea. There's so much congestion."

Then she reconsidered. "On the other hand, it's always great to have a bike lane. It seems like with any situation, you're giving something up."

And, officials note, this is just a pilot program. The paint won't cost anything extra.

"We're not deciding between million-dollar projects," said Hacker, of the regional planning commission. "We're just trying stuff out."

The city will begin evaluating the response after about six weeks. Then, next spring, when the two streets are scheduled to be resurfaced, officials will make a decision about whether to keep the bicycle lanes, go back to two vehicular lanes, or try something else.