From the moment that the Barnes Foundation decided to move to Philadelphia, the arrangement was cast as a perfect marriage of interests. The Barnes would become financially sustainable in a new home on the Benjamin Franklin Parkway. The city would finally get a lively cultural attraction to occupy a primo spot on that great boulevard of dead space.

The $200 million museum design unveiled last week promises to be everything the celebrated art foundation could have desired: refined, serene, uplifting.

But there's one thing it's not, and that's lively.

The disconnect between the objectives of the two parties in this union demonstrates a trait that continues to vex civic architecture. Designers know how to make buildings that dazzle us visually. Yet they're often so intent on satisfying their client's complex organizational needs, they forget about their obligations to city life. The Barnes design, by New York's Tod Williams and Billie Tsien, gets an 'A' in aesthetics and an 'F' in urbanism.

It's not too late to improve the situation, especially at the critical, downtown-facing 20th Street corner, since preliminary site work won't begin until November. In anticipation of the Barnes' opening in 2012, the city is preparing $10 million in streetscape improvements aimed at restoring the primacy of the pedestrian realm along the famously car-centric parkway.

The projects, which range from new sidewalks to a delightful children's park, amount to nothing less than a redistribution of land from the driver to the walker and the bicyclist. They will help the parkway evolve into a real urban place. How ironic it would be if the agent of this momentous change failed to contribute to the parkway's improved street life.

Given the intense controversy that dogged the Barnes' decision to move its renowned collection of Impressionist and modernist art, the architects' strategy for siting the building is understandable. They were committed to replicating the intense, intimate, almost spiritual experience of visiting the original suburban Merion galleries.

So, the linear portion facing the parkway is envisioned as an abstracted version of the Barnes' neoclassical home. And like the original, the new building will be surrounded by lush gardens that insulate visitors from the noisy city.

The design defers to the art on almost every point, and that has cost the Barnes some urbanity. Instead of having the main entrance face the parkway, Williams and Tsien located it around the back. While the rear entrance makes sense within the logic of the design, it means that the architects must find other ways to animate the building where it greets the city.

The Barnes' design doesn't quite ignore the world outside. It flirts with the city by placing a run of French windows on the parkway, so passersby will perceive activity inside. That arrangement should convey a mysterious reserve.

The problem is that the architects, including landscape architect Laurie D. Olin, have not balanced the facade's reticence with something friendlier in the 20th Street gardens.

The corner may be beautifully composed, with an elevated fountain that will be studded with water lilies, but another passive green space is not what Philadelphia needs to draw people to a street that is supposed to be its Champs-Elysees. And the drop-off plaza is vastly overscaled to accommodate the turning radius of the two charter buses that are expected daily. Nothing there will encourage exploration of the surrounding neighborhood, and the inactivity could even discourage development of the tracts of surface parking on Callowhill Street.

Fortunately, the landscape plan for 20th Street is one of the easiest things to fix. For starters, the architects could eliminate the drop-off plaza and replace it with a sidewalk cutout that would allow buses to enter and exit in the same direction. Who needs a turnaround when 20th Street runs only in one direction?

Activating the corner plaza requires more thought. The default solution is a cafe or, better yet, a garden restaurant such as Central Park's Tavern on the Green. Right now, the museum eatery is secreted in the rear.

A stand-alone cafe will be tricky because the Center City District is planning one on the opposite side of Logan Square, as part of a new design for Sister Cities Plaza.

Located in front of the Cathedral Basilica of SS. Peter and Paul, that orphaned piece of the old square will be transformed by the end of 2010 into a charming children's discovery garden, with a pond for toy boats, designed by Studio Bryan Hanes. DIGSAU architects have created an interesting cafe and meeting room that will be another attraction. It's astonishing how much activity the design packs into 1.75 acres.

Other than building a cafe or restaurant on 20th Street, the Barnes' options are few. Fairmount Park, which rules the parkway, objects to visible commerce on the boulevard. But there may be a way out of the dilemma - bicycles.

The 20th Street corner is the perfect spot for a rental kiosk. Some might argue that the humble bicycle is an unlikely companion for an aristocratic art palace such as the Barnes, but remember that the museum intends to adhere to high environmental standards. What's greener than a bike? It sure beats a 26,000-square-foot bus drop-off.

The Barnes' corner is also the entry point to the great Kelly Drive bike path. A short excursion is a perfect way for visitors to relieve museum fatigue - or to visit the parkway's other cultural outposts. To avoid a kiosk that detracts from the Barnes' elegance, the foundation could hold a design competition. And if the proposed structure included a cafe, so much the better.

Fairmount Park also has been thinking about bikes. In March, it will begin reconfiguring the parkway to eliminate one car lane in each direction in the local travel sections. The space it gains will provide land to widen the sidewalks and improve access for bikes. The bike lanes are being shifted to the local sections.

Meanwhile, to make it easier to cross the formidable parkway, Fairmount Park will widen the central median from six to 18 feet to enlarge the comfort zone for pedestrians. By the end of 2010, all the sidewalks and landscape edges will be upgraded to the park's elegant new standard. The long-overdue improvements are a dramatic reflection of the city's shift in priorities: It's not just about moving cars anymore. Pedestrians and bicyclists are finally getting equity on the streets.

Even PennDot is going with the new program. The final piece of the improvements will come when the state begins rebuilding a series of bridges supporting the crossings over the Vine Street Expressway. As part of the project, which could start in 2013 (just as the Barnes opens!), PennDot will cap one of the four ventilation openings in front of the Free Library for another vest-pocket park, on a remnant of Logan Square. It also will add sidewalks on Vine Street.

But the state says that capping the other three openings would be just too costly. Maybe so. But there will never be another chance like this to repair the damage done to the square by the expressway.

The Barnes architects say they envision visitors wending their way through the 20th Street gardens before discovering the museum entrance. That's a lovely idea. But surely the best part of any journey is the distractions we encounter along the way. Let's have more of them.

Contact architecture critic Inga Saffron at 215-854-2213 or isaffron@phillynews.com.