One of the busiest buses in New York City, the Bx12, starts its route at one end of the A train, in Inwood at the very top of Manhattan, and runs across to Co-op City, in the Bronx—the largest housing coöperative in the world. In between, it crosses a lot of places people might want to get on: the 1 train, the 4, the D, the 2, and the 5; the tip of the Bronx Zoo; the bottom of the Botanical Garden; Fordham University; the Metro-North railroad (Hudson Line); and the Bruckner Expressway, an enormous highway designed by Robert Moses, which cuts large swaths of the Bronx off from the water.

The Bx12 is almost always full. On a recent weekday afternoon, large crowds waited at each stop, and people pounded on the back doors when they couldn’t squeeze on. There is no cross-town subway in the Bronx, which is part of the reason that Fordham Road, where the Bx12 often slows to a crawl, is the second-busiest bus corridor in the city. (The first is the M15, which goes up and down First and Second Avenue in Manhattan.) Most New York bus lines don’t collect nearly enough fares to cover their operating costs. The Bx12 comes close.

Soon, the bus might be free. Zohran Mamdani, the Democratic Party’s nominee for mayor, won June’s primary in a landslide, partly on a promise to make every bus route in the city faster—and fare-free. (In 2023, as a state assemblyman, Mamdani had co-led a pilot program that made one bus route in each borough free for a year.) Recently, Andrew Cuomo, who lost the Democratic primary and who is now running as an Independent, announced that he, too, wants to make the bus free, but only for low-income New Yorkers. (Cuomo made the announcement in front of a sign that read “We have problems . . . but nothing we can’t solve. . . .”) Eric Adams, the current mayor, had originally attacked Mamdani’s policy as unrealistic and expensive, but he has started to soften. “I’m not opposed to free buses,” he said earlier this month, in an appearance on a podcast called “Smart Girl Dumb Questions.” He said of Mamdani’s free-bus trial, “When he presented that to me at Gracie Mansion, I said, ‘Wow, that’s a good idea.’ ”

Is it a good idea? 1.3 million people catch the bus every day—roughly forty per cent of the daily subway ridership. People want a lot of things from the bus. They also don’t expect much. Commuters often find themselves waiting for the bus at a low moment—when the train is down, or it’s late at night—and then, it won’t arrive. (Industry experts call this a “ghost bus.”) Occasionally two buses will come at the same time, a phenomenon, known as “bus bunching,” which is extremely complex to model, like fluid dynamics or global supply chains, and depends on intricate traffic flows. The average speed of a Manhattan bus is 6.3 miles per hour, about the pace of a light jog. The fare-evasion rate is at forty-five per cent, according to the M.T.A. (For the subway, it’s only ten per cent.) Since 2008, drivers have been told that they don’t have to enforce the fare.

Danny Pearlstein, the spokesman for the pro-transit group Riders Alliance—which supports the free-bus policy—told me recently that the bus “is a vehicle of last resort.” People rely on it, but they don’t like it. Making it free, he said, would boost ridership and speeds, lead to improved service, and give a financial break to bus riders, who are generally lower income. (Riders Alliance sells a tote bag that says “Real New Yorkers Ride the Bus.”) “The bus is sort of the invisible workhorse of the city,” Pearlstein said, as we sat pressed close together on a crowded Bx12. “Right now, they’re a lifeline, but they could be a lot better.”

The other day, at a stop on East Fordham Road and Southern Boulevard, Leslie Delgado was trying to head west. “I don’t think I’ve ever been on a Bx12 that was empty,” Delgado said. She was wearing a bright-yellow T-shirt, and the doors of the express Bx12 had just closed on her because it was too packed to get on. Delgado takes the bus every weekday from her home in the west Bronx to her work as an outdoor educator. As she waited for the next one, I brought up Mamdani’s proposal. “I think it’s great,” she said. “I feel like true New Yorkers know that they’re free—it’s just about accessibility.”

A Bx12 was shuttling toward us, a local, set to stop every three or four blocks, and Delgado chose not to take it. “I like the bus,” Delgado said. “I think there needs to be more of them.” I asked whether she was concerned that making the bus free could result in less bus funding. “Yes, but there’s so much more money going to stuff like cops, and from what I’ve seen they just kind of stand around,” she said. Then she stuck her head out. Two express buses had pulled up at once; she hopped on the first one.

What would happen if the bus became free? Most experts I spoke to were extremely reluctant to speculate. Still, there are a few things that they agreed on. Ridership would go up. “Typically, when something is free, people will take more of it,” Ana Champeny, vice-president for research at the Citizens Budget Commission, a nonpartisan fiscal watchdog, said. Subway habits could change. (A report from the N.Y.C. Independent Budget Office, a nonpartisan government agency, estimated that four per cent of subway rides would switch to bus rides.) Commuters would probably start taking the bus on shorter trips. It’s highly likely that people would walk less.

Speed isn’t guaranteed. Passengers could begin to board from all doors, which would make things faster. But increased crowds might slow it all down. During the free-bus trial, ridership on each of the free lines surged between twenty-two and forty-six per cent, but speeds dipped slightly, by 2.2 per cent on average, potentially because the efficiency gains of faster boarding were cancelled out by delays created by more demand. “Everyone’s asking this question,” Emily Pramik, a lead transportation analyst at the Independent Budget Office, said. “Theoretically, making buses free could reduce what’s called dwell time, which is the time that a bus spends at a bus stop taking on passengers.” Boston is currently trialling free buses, and data shows less dwell time; New York’s trial showed more. Traffic is really the big issue. (The problem, as it always is in New York, is other people.) “It might be faster,” Adam Schmidt, a transit expert at the Citizens Budget Commission, said. “It might not be.” Pramik said, “I have to tell you, I don’t know.”

People on the bus would probably become nicer: data from the free-bus trial showed that assaults on drivers dropped. Would free buses lead to more homeless people using the bus for shelter? Not really, David Giffen, the executive director of the Coalition for the Homeless, told me. “Most people who are sleeping unsheltered, they prefer to find places where they can lie down—buses are not ideal places to get rest.”

How much would it cost? A report prepared by the Independent Budget Office, in 2023, projected an annual price tag of six hundred and fifty-two million dollars. What else costs the city six hundred and fifty-two million dollars? It’s thirty-nine days of running the subway, or around two hundred and fifty days of trash collection and street cleaning, or employing thirty-three hundred N.Y.P.D. officers for a year. It would also cover just three per cent of the M.T.A.’s 2022 annual operating expenses—eleven days. But the cost would likely be higher. The report estimated that the M.T.A. collected about seven hundred million dollars in 2022 from bus fares. In 2025, the M.T.A. budget aims to collect eight hundred and fifty million, and, in 2026, the fare is set to rise to three dollars. (“The price hikes compared to the service that we get, it doesn’t equate,” Delgado told me, at the bus stop.)

The six-hundred-and-fifty-two-million-dollar figure also doesn’t factor in the cost of running extra buses if ridership were to explode. Pramik, who was one of the authors of the I.B.O. report, told me that, at least in 2023, the bus system could take a ridership bump of twenty per cent without extra expenses. That would have brought ridership closer to pre-pandemic levels. But there’s a tipping point. Champeny, of the C.B.C., said, “For a while, the extra cost is going to be zero—until you tip, and you need another bus. And then you have this big jump.”

Charles Komanoff, a transit expert often cited by the Mamdani campaign, estimates that free buses would generate six hundred and seventy million dollars in economic benefit from saving people time. (Komanoff also predicts a 0.01-per-cent reduction in “all-cause mortality”—“two fewer deaths per year”—due to improved health caused by an uptick in cycling prompted by fewer motor vehicles on the road.)

Finally, there’s the six-hundred-million-dollar question of who pays. The city of New York does not control the M.T.A.’s budget. The money for free buses would have to be found through negotiation with Albany and Governor Kathy Hochul. “There’s plenty of money in New York to support free buses if our political leaders prioritize it,” Pearlstein, of Riders Alliance, said. The current administration disagrees. “Mayors can’t do that,” Adams said on “Smart Girl Dumb Questions.” “The governor already said, I’m not signing off on that.”