The Sixers want to model their arena on Boston’s TD Garden. I went there to see how it works.

Since the Sixers first began pitching Philadelphia on the idea of putting an arena at 11th and Market, the team’s front man, David Adelman, has regularly portrayed Philadelphia as an outlier in the world of professional basketball. Of the 30 NBA teams, he says, 28 play in downtown arenas. Only “us and Chicago” lack downtown venues, he told the Design Advocacy Group on Oct. 19.

Adelman has repeated this 28-out-of-30 statistic in community meeting after community meeting. He cited it in an interview with 6abc in July and in an article that appeared in the Philadelphia Business Journal in August. Sometimes he varies the phrasing a bit, referring to the top 20 media markets instead of the NBA’s 30 hometowns, but then follows up by predicting that “Philadelphia will be 29.” Each time Adelman and I have talked, he has used the numbers to buttress the Sixers’ case for locating an arena in the dense heart of Philadelphia, two blocks from City Hall.

During those interviews, Adelman has frequently urged me to visit Boston, the American city that most resembles Philadelphia physically, to see how its arena, TD Garden, fits into its surroundings.

“As a Sixers owner, I obviously hate the Celtics,” he often says. But he is unabashed in his enthusiasm for their arena, and the new development that surrounds it. TD Garden, he told me, is a model for what the Sixers want to accomplish in Philadelphia.

So I ventured into enemy territory to see for myself.

And there I made a surprising discovery: TD Garden is not downtown.

While TD Garden is in a relatively dense neighborhood, the area is really on the fringes of downtown in a former warehouse district. In the last few years, several skyscrapers — part of a development called Hub on Causeway — have finally started to go up on the adjacent surface parking lots. Yet TD Garden, which opened in 1995, still has plenty of open space on its perimeter. The arena faces a charming Charles River boat basin on one side and the deep cut of I-93 on another.

Like the Fashion District, where the Sixers hope to put their arena, TD Garden sits on top of a commuter rail hub, North Station. But in Boston’s geography, North Station occupies the place here held by 30th Street Station’s rail yards. I was staying in Boston’s Downtown Crossing — right by Macy’s, incidentally — and it took me 15 minutes to walk to the Garden.

Obviously, no two American cities are arranged the same way. But the differences between the urban conditions at 11th and Market and those at TD Garden are not minor and should be considered when the Sixers present their project to Philadelphia’s Civic Design Review board on Dec. 18.

We know that even the most sensitively designed basketball arena is an urban beast. These large gathering places invariably dominate the street, yet rarely support more than 150 events a year. And when nothing is going on — as was the case on the day I visited TD Garden — people have no incentive to visit.

Although the Sixers might succeed in putting in a few retail tenants on the ground floor of 76 Place, it’s unlikely to become a shopping destination. We only have to look a little north, at the three-block-long Pennsylvania Convention Center, to understand how an infrequently used building can drain the life out of a neighborhood.

Just as every city’s urban form is different, so is the area called downtown. Philadelphia is that rare American city with a nearly intact downtown, which we call, in our quaint fashion, Center City. Our downtown is a place where modern skyscrapers and diminutive rowhouses, early 20th-century commercial buildings and humble storefronts, stately churches and magnificent government offices are all jammed together in a walkable, colonial-era street grid. We were mixed-use before mixed-use was cool.

Once I understood that TD Garden wasn’t truly downtown, I began looking at several other arenas that the Sixers have highlighted as inspiration for 76 Place. Clearly, the locations of Manhattan’s Madison Square Garden and Brooklyn’s Barclay Center can be considered downtown. But San Francisco’s Chase Center? It’s in Mission Bay, not downtown, and overlooks Bayfront Park. Detroit’s Little Caesars arena? Not downtown. Denver’s Ball Arena? Not downtown.

What struck me is that many NBA arenas tend to be just outside the highway ring that encircles — and simultaneously walls off — the downtown. The Sixers’ arena at 11th and Market would be unusual in that it would be inside that highway ring and more than a half-mile from the nearest highway exit.

Although TD Garden is not downtown, what’s happened along Causeway Street is definitely impressive. Over the last decade, Gensler (the same architecture firm that is designing 76 Place) has created a row of elegant buildings — a 500-foot Verizon office tower, hotel, and apartment building — to screen the arena. There are bars and restaurants in the complex, along with an AMC movie theater. A Boston city planner who helped shape the zoning for the site told me that the area now has “the best bar scene in Boston.”

Yet one feature of urban life was in surprisingly short supply: actual retail shops. You know, the kind that offer the sort of consumer goods you can touch and take home. I did see one store selling clothing with team logos near the entrance to North Station, as well as a supermarket, but very little else in the category that retail experts call “soft goods.” CVS operates a store across the street from the Garden, in the precise urban niche as the one at 10th and Market, across from Fashion District.

By contrast, I counted 30 occupied lots just in the Fashion District’s westernmost block, the one where the Sixers want to build their arena. That includes a two-story Primark, Forever 21, Aéropostale, an Ulta beauty store, as well as an AMC theater, bowling alley, an immersive art exhibit called Wonderspaces, and several food vendors at concourse level. All would have to be relocated for the Sixers to construct the arena.

John Gallery, a former Philadelphia official who was deeply involved in planning the original version of the Fashion District in the 1960s, believes the arena would be the “end of retail on Market Street.”

No one doubts that the shopping mall faces intense economic headwinds. As of June, occupancy in the mall was only 79%, and the owners say they could default on their loan obligations in January. And Market Street has been stuck with surface parking lots at Eighth and 13th Streets for decades.

But the corridor can also boast a new, billion-dollar development called East Market between 11th and 12th. With two apartment towers, an office building, hotel, supermarket, T.J. Maxx store, and several restaurants, it’s just as impressive as the Hub at TD Garden. And next year, Jefferson Hospital will open a state-of-the-art outpatient facility called the Honickman Center.

A recent study by the Center City District suggested that 76 Place could spur similar mixed-use developments elsewhere on Market Street, yet offered no evidence of cause and effect. There’s no reason such mixed-use development can’t occur without an arena.

Once I had a better sense of TD Garden’s position in Boston’s geography, I wondered why there has been no discussion about locating 76 Place over the rail yards at 30th Street.

For decades, Philadelphia has dreamed of decking over the 60-plus acres that stretch along the Schuylkill waterfront, from Arch to Spring Garden Street. Before the city created the South Philadelphia Sports Complex, planners floated the idea of building a sports stadium over the tracks. Constructing a deck over an active rail line proved too expensive at that time, and the proposal never went anywhere.

But a lot has changed in the last 60 years. Brandywine Realty Trust’s Schuylkill Yards has transformed the blocks around the train station into a bustling hub for scientific research. Brandywine just completed its first residential building, the Avira, and three large lab buildings, including one by Gattuso on the Drexel University campus, are under construction. An arena could add an entertainment component to the mix.

Finding a good site won’t be easy. In my view, the best available location is the deck at 29th and Arch Streets, just east of the Cira tower, which is now used as a parking lot. At two acres, the deck is too small for the Sixers’ needs. But if the platform were extended east over the tracks, the Sixers would gain a high-visibility perch with stunning views of the Center City skyline. Because the location is so far from a residential or retail district, it hardly matters that the building would sit empty 200 days a year.

Clearly, this would be an expensive undertaking and require some engineering gymnastics. When Brandywine erected the FMC Tower, Amtrak had to turn off the electricity on the Northeast corridor while the building’s foundations were constructed. Brandywine’s contractors could work only in the wee hours, roughly from midnight to 4 a.m., forcing the company to pay its workers triple-time.

The Sixers actually face a similar challenge at the Fashion District, where they would have to build over the electrified tracks at SEPTA’s Jefferson Station. In both cases, this arena would be expensive and challenging to construct. But building on a tight urban site such as the Market Street location would be even more fraught. Commuters and Chinatown businesses would have to endure enormous disruptions.

If Philadelphia were truly committed to an independent review, it would be city planners — not the Sixers — leading the search for a new arena location. The city has not undertaken a comprehensive master plan for Market Street since 2009, when a casino was proposed as a replacement for a block of the Fashion District. We need a citywide conversation about Market Street’s future: Can it remain a retail corridor? Should it become an entertainment district? Or would we be better off if it evolved into a residential boulevard?

Meanwhile, there’s another transit-accessible site where the Sixers should consider building a new arena. It’s not downtown, but just outside the central core. It’s called the Sports Complex.