The plan was to transform two of New York City’s busiest crossings for cars into “Bridges for the People,” an idea that Bill de Blasio, the mayor at the time, said would help New Yorkers exit “the era of fossil fuels.”

He promised to remove one vehicle lane apiece from the Brooklyn and Queensboro Bridges to accommodate a pandemic-spurred boom in walking and cycling, a sign of just how far the city had moved away from the car culture that has long dominated its streets.

Four years later, only the Brooklyn Bridge is friendlier to pedestrians. The Queensboro Bridge remains the only city-owned East River bridge without separate paths for pedestrians and cyclists, jamming them instead into a single, overcrowded lane.

Last week, there was hope that the Queensboro Bridge’s time had finally come. City transportation officials were poised to hit send on a news release announcing the opening of a new pedestrian path on the bridge’s southern flank, according to several people familiar with the plan.

The release, which The New York Times obtained, was headlined, “Bridges for People,” and said the project would be “the first bike and pedestrian upgrades to the bridge” since 1979, when the existing walking and bike path was carved from the outer northbound roadway.

City transportation officials even went so far as to invite Councilwoman Julie Won of Queens to a ribbon-cutting ceremony on Sunday, March 16. Assemblyman Zohran Mamdani of Queens provided a quote for the news release.

But then the mayor’s office scuttled the plan, to the dismay and frustration of the project’s supporters.

Although a city official said the event had been added to City Hall’s internal event-tracker in late February, Kayla Mamelak Altus, a spokeswoman for Mayor Eric Adams, said city transportation officials had failed to brief him and his aides about the project before proceeding. She said the mayor’s office had requested data so it can assess how the project may affect traffic in Manhattan and Queens.

She said that “nothing has been delayed, and the mayor and City Hall must be provided a full briefing on how the agency plans to roll this out smoothly and ensure New Yorkers can continue to get to where they need to go efficiently.”

The Brooklyn Bridge was reconfigured in September 2021, before Mr. de Blasio’s term ended. The Queensboro Bridge, however, was undergoing rehabilitation, and Mr. Adams, who has described himself as a cyclist, inherited the project. (Mr. de Blasio did not respond to a request for comment.)

In recent decades, New York City has increasingly carved out room for pedestrians and cyclists on its streets. In December, the Transportation Department celebrated an “all-time high” for bike ridership across the four East River bridges. At the same time, the department committed to doubling cycling and pedestrian space on the Queensboro Bridge in 2025.

Mr. Adams, a self-described working class mayor, has been criticized by transportation advocates for not doing enough for city residents who do not commute by car, which is most of them. His office watered down plans to improve bus speeds along Fordham Road in the Bronx, the busiest bus route in the poorest borough. City officials also scaled back efforts to make McGuinness Boulevard in Brooklyn friendlier to cyclists.

And though it built nearly 90 miles of protected bike lanes in the past three years, an improvement on the de Blasio administration’s final years, the Adams administration has failed to meet the ambitious bus- and bike-lane requirements in the city’s “Streets Plan” law — requirements the transportation commissioner recently told the City Council were “not realistic.”

With Mr. Adams’s poll numbers in the tank and the Democratic primary for mayor just three months away, critics have wondered whether the mayor might be trying to quash a project that could draw the ire of drivers and possibly also President Trump, who disdains bike lanes and with whom Mr. Adams has developed a mutually beneficial relationship.

“LOL! What a ridiculous reach by your unnamed ‘critics,’” Ms. Mamelak Altus said in an email.

Mr. Mamdani, who is running to unseat Mr. Adams, said the repeated delays in opening the Queensboro Bridge path were infuriating.

“This administration time and again has politicized basic street safety projects, intervening at the last minute and putting New Yorkers at risk for completely arbitrary political decisions,” he said.

The back-and-forth over the Queensboro Bridge path also suggests a heightened level of disorganization within a City Hall that has experienced substantial turnover since half of Mr. Adams’s deputy mayors resigned in February.

The bridge, which was completed in 1909, once carried trolleys alongside cars. The crossing is now used by 170,000 vehicles a day, officials said.

A growing number of cyclists and pedestrians must squeeze onto the 11-foot-wide lane on the bridge’s outer northbound roadway. More than 7,100 cyclists and 2,700 pedestrians use the path every day.

There have been 19 crashes reported on the shared path since 2022, according to city officials. On the Manhattan Bridge, which has distinct pedestrian and cycling lanes, there were 14 in the same period. The city did not provide similar statistics for the two other East River bridges.

In October, Daniel Bach, a lawyer, was jogging over the Queensboro Bridge when he was hit by a scooter. He ended up in intensive care with fractured eye sockets and a broken nose.

“Bottom line, clearly, cyclists and scooters should be on the other side of the Queensboro Bridge and not sharing that little path with the runners,” said Mr. Bach, 62, who lives in Long Island City, Queens.

Ms. Won said city officials had contacted her office on March 10 to invite her to the ribbon-cutting six days later. They told members of her staff that a news release would be issued on March 12.

But four days after the initial contact, city officials told Ms. Won’s office that the ribbon-cutting would not happen, she said. The delay was first reported by the transportation-focused website Streetsblog.

At a City Council hearing this week, Ms. Won pressed Transportation Department officials about the status of the Queensboro Bridge project.

“We were told by D.O.T. that the construction was complete,” Ms. Won said. “So did they misspeak?”

In response, a transportation official disputed that characterization but could not provide a precise timeline for when it would be finished.

“It will happen this year,” said Ydanis Rodriguez, the transportation commissioner. “Very soon.”

On a recent afternoon, the new walkway beckoned from behind a fence, while on the existing walkway, a skateboarder weaving around a stream of pedestrians clipped one.

Corey Zeigler, a cyclist, longs for more space on the bridge. Not long ago, Mr. Ziegler, a 32-year-old construction worker, crashed on the Queens end of the bridge and nearly lost his left ear.

An Astoria native, he has watched the area’s skyline become crowded with high-rise apartment buildings. That has made the neighborhood, and the shared pedestrian and bike path on the Queensboro Bridge, “10 times more crowded and dangerous” than it was a decade ago, he said.

“What is the reason for it being held up?” he asked. “If it’s political, we shouldn’t stand for it.”

Nate Schweber contributed reporting.