By MICHAEL M. GRYNBAUM   Published: September 5, 2010

It is Manhattan’s most famous thoroughfare, known around the world for its theater marquees and giant Macy’s. It has come to symbolize the outsize aspirations and swagger of New York.

But under the Bloomberg administration, Broadway has been transformed, from a grand avenue that ferried automobiles on a scenic route through Midtown to a narrow passageway with barely more room for cars than a sleepy street in Greenwich Village.

In two years, roughly three and a half miles of the street’s moving lanes have vanished — nearly half of the total between Columbus Circle and Union Square — and in some spots automobile traffic has dropped by a third. Dozens of parking spaces are gone, replaced by bicycle lanes and pedestrian picnic areas. For the first time in New York’s modern era, Broadway no longer offers a continuous path from the Bronx to the Battery.

The metamorphosis is set to continue after Labor Day, when the city will remove one of the two remaining driving lanes in the five blocks north of 18th Street by Union Square. By autumn, nearly all of once-bustling Broadway from 33rd to 17th Streets will be reduced to a single lane of moving traffic, save for an occasional stub for left turns.

And city officials are considering a further expansion of the pedestrian space along the corridor, although they say there is no current plan to close it to cars entirely.

The changes — perhaps the swiftest re-engineering of a major New York roadway ever — have made the street more palatable to pedestrians and bicyclists, making it a microcosm of a broader plan by the city to reallocate road space traditionally used by cars. Transportation officials say that accidents have decreased and nearby avenues in Midtown are less backed up.

“It’s like a green ribbon that goes from 59th Street down to 14th Street now,” Janette Sadik-Khan, the city’s transportation commissioner, said of the new Broadway. “Traffic is better, injuries are way down. We are accommodating thousands of more pedestrians.”

But many drivers remain hostile to what some say has amounted to a tacit decommissioning of Broadway as a major thoroughfare. The street is increasingly shunned by drivers. Compared with a year ago, the number of vehicles using Broadway between Columbus Circle and Times Square has gone down about 25 percent, the city says. And in the morning rush, traffic on Broadway passing 23rd Street has fallen 30 percent since 2008.

“I know they’re trying to beautify the city, but it’s killing the drivers,” said Gus Salcedo, 40, a daily car commuter from Queens who was parked on Broadway at 33rd Street the other day. “It’s frustrating. They don’t want you to drive into the city.”

Traffic planners at the city’s Department of Transportation say that less automobile traffic on Broadway is, in fact, a symbol of success, noting that the street’s awkward three-way intersections with other avenues created gridlock. “The mayor asked us to take a look at what we could to untangle the Gordian knot of traffic in Midtown,” Ms. Sadik-Khan said. “We’re making the network work like it was supposed to.”

And officials do not flinch in asserting the proper role they believe Broadway should play for cars in the Midtown street grid. “Broadway is now more of a local street,” Ryan Russo, an assistant transportation commissioner, told a Midtown community group this spring.

Much of the public attention to the changes on Broadway has focused on one element: the pedestrian plazas that banned cars entirely from parts of Times and Herald Squares, creating open-air concrete parks in the center of Manhattan, complete with brightly hued beach furniture.

But with less fanfare, city officials gradually stripped away vehicular lanes, extended sidewalks into the road and put parking lanes in the middle of the street. European-style spaces allow pedestrians to sit, chat and dine as cars whiz by, earning praise from business groups about the aesthetic improvements to a formerly exhaust-filled street.

“You do these things incrementally, and over time they build,” said Jeffrey Zupan, the senior fellow for transportation at the Regional Plan Association. “It’s given people a different feeling about walking in the city, that the pedestrian isn’t a second-class citizen who has to always be on the lookout of getting run over.”

In pursuing a policy that discourages automobiles from using the street, traffic planners see themselves as issuing a corrective to history: They say the diagonal of Broadway should never have been allowed to cut a path across the orderly right angles of the Midtown street grid. The resulting three-way intersections can slow down cars and tie up the broader system.

“It sounds counterintuitive that removing a street can make things better. But it was a mistake in 1811 when they left Broadway in as a traffic street,” said Samuel I. Schwartz, the éminence grise of the city’s traffic circles.

In fact, the leaders behind the Commissioners’ Plan of 1811, which proposed the famous grid system, were well aware of a diagonal’s stymieing effect, according to Michael Miscione, the Manhattan borough historian.

The commissioners’ original plan called for Broadway to run north in a straight line from the Battery to 23rd Street, and come to an end in a never-built military exercise ground known as the Parade.

But New Yorkers, then as now, were no fans of change. A group of farmers led by Henry Brevoort Sr. objected to the grid plan, which had Broadway running through their orchards. The city agreed in 1815 to bend Broadway at Mr. Brevoort’s farm, at East 10th Street, where the street still curves today.

Meanwhile, planners were finding it hard to persuade the city’s business establishment to give up the Bloomingdale Road, a well-trod shipping route to Albany that happened to cut an inconvenient diagonal path across what is now considered Midtown. The result: Broadway was extended to replace Bloomingdale Road, creating the familiar crisscrossing route seen today, Mr. Miscione said.

Traffic experts have tried to tackle Broadway’s shortcomings over the years, making the street one-way southbound below Central Park in the 1950s and 1960s. But that process took about a decade, a snail to the hare of Mayor Michael R. Bloomberg’s administration.

The golden ideal, for some of the city’s more progressive transportation planners, would be to ban cars altogether from the Midtown length of Broadway. The city experimented with banning vehicles from parts of Madison Avenue in the early 1970s, but the idea never stuck. Some officials see Broadway headed in this direction.

“I think it was their strategy to introduce this piecemeal, see if it worked, and then go further,” said one well-connected city official who speaks frequently with Ms. Sadik-Khan and insisted on anonymity to preserve a relationship with the commissioner.

City planners have argued that decreased traffic volumes, caused in part by their own changes to Broadway, can justify the further removal of lanes along the road. In presenting the city’s plans to remove another lane of traffic north of Union Square, Mr. Russo, the assistant commissioner, told the local community board that they would not have to worry about more congestion. “Traffic is down 30, 40, 50 percent in the area,” Mr. Russo told the group. “Those trips aren’t happening now.”

Asked in an interview if she had plans to ban automobiles entirely from Broadway, Ms. Sadik-Khan said such a plan would not be “realistic” right now.

Pressed about her plans, she said that as the street continued to evolve, Broadway would remain accessible. “Behind the wheel of an automobile, behind the wheel of a bike, behind the wheel of a bus,” she said, “you’ll still be able to wheel down Broadway.”