The Northern Avenue Bridge is being replaced, and the city is debating just who will be allowed to use it

“There are a lot of things we need to figure out here,” said Chris Osgood, Boston’s chief of streets. “But there is universal interest in having a Northern Avenue Bridge again.”

The swinging bridge was built in 1908, with road and rail lines connecting downtown with the old industrial waterfront. In 1997, it was closed to vehicle traffic and spent the next 17 years as a quirky, rusty pedestrian pathway between downtown and a neighborhood that until then was still mostly parking lots.

Then in 2014, city engineers closed the bridge, citing deteriorating floorboards, and left it in the open position. There it has sat, while the city studies whether the structure — or portions of it — should be retained in a new bridge, or replaced altogether. A decision and preliminary design are expected later this year.

The Seaport is now a booming business district, but getting in and out is a daily challenge for thousands of commuters. Many drive to work over the neighboring Moakley Bridge, or older spans at Congress and Summer Streets.

All that traffic has some pushing for car lanes on Northern Avenue, including Representative Stephen Lynch. The South Boston Democrat, who secured $9.4 million in federal funding for the project, has called for “maximum utility” in the new bridge.

“In order to address the public safety needs of my current and future residential and commercial neighbors in the South Boston Seaport area, the plans for the Northern Avenue Bridge will require the capacity to carry vehicular traffic,” said Lynch. “That fact does not vitiate the need for a bridge structure that is also friendly to pedestrians and bicyclists.”

Safety is a concern, too, for the judges and other federal officials who work inside the Moakley US Courthouse, which sits at the eastern end of the bridge. Susan Goldberg, the regional circuit executive for the federal courts, said a new bridge must at least have access for fire trucks, ambulances, and other emergency vehicles.

“We support pedestrian and bike transportation, but we also want to see at least one lane of vehicle traffic for emergencies,” Goldberg said. “Our primary concern is emergency access.”

The city, meanwhile, has received mixed advice from studies it has helped fund. A 2015 study of transportation in the Seaport, authored by the business group A Better City and paid for by city and state agencies, concluded that a replacement bridge should have a lane for “peak directional vehicle traffic” — a one-way lane that would flow into the Seaport during the morning rush, and out in the afternoon commute.

But another study, paid for by the city and released this summer by consulting firm AECOM, concluded that allowing cars onto the new bridge would do little to ease traffic, and could simply create new backups on Northern and Atlantic avenues.

Because of its location, there would be no benefit for drivers heading into the Seaport over Northern Avenue in the morning and minimal gain during the outbound afternoon commute, said Jay Doyle, a consultant with AECOM, at a meeting of the city task force in late July. Traffic engineers already grade the intersection of Northern and Atlantic avenues an “F” — as in failing — and, said Doyle, with a new bridge and car access “it would still be an F.”

A better option, Doyle suggested, may be a travel lane just for buses, and possibly car-poolers. Routing buses over Northern Avenue instead of the crowded Moakley Bridge could shave four minutes off the trip between the Seaport and North Station, AECOM estimated. That, in turn, could prompt more commuters to give up driving to work.

“There are 4,000 to 5,000 people who work in the Seaport who live along I-93 North,” added Rick Dimino, chief executive of A Better City. “A faster link to North Station will attract a number of those people to transit.”

But the Seaport and neighboring Fort Point are also home to a growing residential population. They routinely walk or bike across the Fort Point Channel and would rather not use the auto-centric Moakley to do so.

Sara McCammond of the Fort Point Neighborhood Association said the “overwhelming majority of people” at a recent association meeting supported a bridge designed chiefly with pedestrians and cyclists in mind. No cars.

Bud Ris, former chief executive of the New England Aquarium and member of the task force, said the need for better pedestrian and cyclist access is obvious.

“Every afternoon there are massive numbers of people crossing that bridge at Seaport Boulevard,” Ris said. “The more we can encourage bikes and pedestrians, the better.”

Then there’s the money question. Walsh has pledged $46 million of city funds toward the bridge. But with a cost running about double that, the project would likely require federal dollars, or contributions from nearby businesses.

While a car bridge is not likely to cost much more, if any at all, than one for pedestrians and emergency vehicles, some transportation experts say it may have an easier time receiving federal highway funds (though there are programs that finance transit and bike bridges, too). Private developers will probably be asked for funds, too. WS Development has already pledged $2 million toward the project as part of permitting for its Seaport Square project. City officials may hit up other businesses in the area.

If anything, said Stacy Thompson, executive director of the Livable Streets Alliance, there is a risk of asking too much out of the project. If the new bridge is, all at once, for people, transit, and cars, then it might just wind up pleasing no one.

“We need to figure out what we’re trying to solve, then ask is this the best solution?” Thompson said. “There are so many questions and concerns about how we get around this city. One tiny little bridge isn’t going to solve all of them.”

Tim Logan can be reached at tim.logan@globe.com. Follow him on Twitter @bytimlogan.