Philadelphia didn't need Bicycling magazine to confirm that it is one of America's best biking cities (No. 17 on its 2012 list). You can see it every day on the streets: The steady stream of commuters sluicing down Center City's bike lanes. The tangle of bikes hitched to U-shaped racks and bike corrals. (More, please.) The proliferation of neighborhood bike shops.

Philadelphia probably could have ranked higher in the magazine's esteem if it had a bike-sharing program, like most of the list's top 20 cities. You can now find cheap, on-street bike rentals in more than 135 places around the world, many of them with worse weather and hillier streets than Philadelphia. Yet the city has remained strangely ambivalent toward the concept, even as private bikes have become a popular transit option within the city.

But the sight of Mayor Nutter tooling around Rittenhouse Square last week on a canary-yellow cruiser suggests Philadelphia is finally ready to commit. To show the city's seriousness, his Transportation Department organized a daylong bike-sharing demonstration with three top vendors, supplying a docking-station's worth of bikes in paint-box colors.

The mayor is promising $3 million toward the project, which would cost $8 million to $11 million to launch. A bike-share system, which allows people to rent a bike for a nominal fee at one docking station and drop it off at another, could be ready to roll as early as next summer.

The initial phase calls for building a network of up to 120 docking stations with 1,500 bikes in the densest part of the city - roughly, from Temple University south to Federal Street, with a prong extending west into University City. Since you can't install that quantity of hardware without making a sizable impact on the streetscape, it's not too early to start worrying - er, thinking - about where to locate the docking stations.

Despite bike-sharing's growing popularity in American cities, there is still deep concern about how it will work in Philadelphia. With its colonial-era street grid and skimpy sidewalks, we already have a fierce competition for primacy among pedestrians, motorists, and regular bicyclists. In the last few years, the city's sidewalks have also become cluttered with "street furniture," like parking kiosks and Big Belly trash bins. What happens when hundreds of bike-share riders join the crowd?

The space concerns aren't unique to Philadelphia. In New York, which expects to inaugurate its bike-share system later this month, siting choices for docking stations have provoked bruising battles - and New York's sidewalks are generally much wider than Philadelphia's.

Fortunately, the Mayor's Office of Transportation and Utilities has already begun to formulate ground rules specific to Philadelphia.

In contrast to other cities, they've excluded sidewalks as potential locations, Andrew Stober, the project's point person, told me. Ditto for sites inside Center City's four main squares. Meanwhile, to avoid antagonizing motorists and business owners, Stober says the city has decided not to place docking stations in parking spaces during Phase 1, except as a last resort.

No sidewalks. No street spots. What's left?

A lot more places than you would think, Stober insists.

Next month, the city will release a report on how to implement bike-sharing, Stober says. His office has already identified 130 locations that fit the city's strict siting criteria. Many proposed docking stations could be installed on private property, such as the Comcast tower plaza, when the owner agrees, or on the aprons fronting city museums. Stober says the city plans to solicit public comment for every site.

Ideally, the spots need to be close to the street, so riders don't have to wheel the bikes across the sidewalk. Because bike-sharing is seen as a form of transit, the city wants docks at all the major stations.

The Porch on Market Street would be a perfect spot for a dock serving 30th Street Station. LOVE Park and the Penn Center walkways could provide bikes convenient to Suburban Station.

Since the docks need to be easily visible, that rules out parking garages as potential sites.

The establishment of these rules indicates the city wants to be sensitive to the various constituencies who use city streets. It's right to ban the bikes from the green refuges like Rittenhouse Square. Even so, winning public approval for the less-intrusive sites will require skilled diplomacy.

Finding good sites is just the beginning. Even many people who are pro-cycling don't fully get bike-sharing. After all, it doesn't cost much to acquire a serviceable bike to cruise around town. (On the other hand, it's nice not to have to worry about storage, repairs, or theft.) And do we really want tourists and people who aren't used to riding in city conditions careening up Market Street at rush hour?

Then there's the helmet problem. No bike-sharing system has found a way to dispense the protection.

Advocates respond that there have been no fatalities in any bike-share city. They also make the case that bike-sharing is a form of public transit that supplements other modes.

For me, that is the strongest argument in bike-sharing's favor. In Washington, which has what is considered the best bike-share program in the country, locals account for more trips than tourists. Someone arriving at 30th Street Station will be able to grab a bike and zip over to the University of Pennsylvania, instead of waiting for a bus or taxi.

The promise of bike-sharing is that it could reduce the need for a car in the city. All the infrastructure needed to accommodate and store cars consumes vasts amount of real estate that could be put to livelier uses. Compared with housing our cars, finding sites for 120 docking stations should be nothing.