Starting Sunday, most drivers will have to pay $9 to enter the busiest part of Manhattan. That much is clear.

But almost everything else about New York City’s congestion-pricing plan, the first of its kind in the United States, continues to be fiercely debated.

Transportation, business and civic leaders, as well as long-suffering subway and bus riders, consider the tolling plan a long-overdue step toward unclogging the city’s gridlocked streets, raising billions of dollars for an aging transit system and encouraging a more sustainable future with fewer cars.

“Congestion pricing will finally tackle the gridlock that is slowing down emergency vehicles, polluting air and wasting people’s time in traffic,” said John J. McCarthy, the chief of policy and external relations for the Metropolitan Transportation Authority, which will oversee the program.

But suburban commuters, residents of the city’s so-called transit deserts and public officials of both parties say congestion pricing will do little to reduce traffic while punishing drivers from outside Manhattan with few other travel options. These critics have called the tolling plan a money-grab by the M.T.A., a state agency with a history of financial problems.

“This is just simply a misguided policy,” said Ed Day, the Rockland County executive. He has sued to halt the program, which, he said, “raises serious questions about fairness, priorities and accountability.”

The New York program is being closely followed by officials and advocates in other U.S. cities who are grappling with their own traffic problems in a country where the car is king. Several cities, including Washington and San Francisco, explored the concept before the coronavirus pandemic interrupted those efforts.

Congestion pricing is being introduced at a time when New York City’s streets are more clogged than ever. From Fifth Avenue to the Brooklyn-Queens Expressway, traffic has rebounded sharply after largely disappearing during the depths of the pandemic.

The city’s traffic is now so thick that New York was named the world’s most congested city in a 2023 traffic scorecard compiled by the transportation data analytics firm INRIX, beating out London, Paris and Mexico City.

Drivers lost 101 hours on average sitting in traffic in New York that year, more than double the national average of 42 hours, according to the scorecard. All that idle time translated to $1,762 per driver in lost wages, productivity and other costs, and a $9.1 billion overall loss for the city.

Samuel I. Schwartz, a former city traffic commissioner who supports congestion pricing, said that any improvement in traffic would be welcome. Within the congestion zone, the average travel speed has dropped to under 7 miles an hour for the first time since records were kept in the 1970s, he said. The slowest traffic crawls along at just 4.7 miles per hour in Midtown.

“Traffic is worse than it’s ever been,” he said.

William Vickrey, a Columbia University professor and winner of the Nobel in economic sciences, came up with the idea for congestion pricing in the 1950s. But it has languished in New York even as traffic-choked cities around the world, including London, Stockholm and Singapore, embraced it.

The idea gained momentum in New York briefly in 2007 when Michael R. Bloomberg, the mayor at the time, unveiled a congestion-pricing plan, only to see it falter in the State Legislature. A decade later, Gov. Andrew M. Cuomo revisited it amid a crisis in subway service. The tolling plan was finally approved as part of the 2019 state budget.

Shortly before the plan was to start in June, Governor Kathy Hochul, a Democrat, delayed it, saying the tolls could hurt the city’s economy. Some critics said the plan, which polls showed was broadly unpopular, would hurt Democratic candidates in the November election.

Ms. Hochul, under pressure from transit advocates, revived congestion pricing in November. To make the tolls more palatable, she slashed them 40 percent across the board.

Most passenger cars will now have to pay $9 to enter Manhattan south of 60th Street at peak hours, rather than the original $15. Small trucks will have to pay $14.40; large trucks, $21.60. Discounted rates will be offered overnight when there is less traffic.

M.T.A. leaders expect the new tolls to help generate $15 billion through bond financing that will pay for a long list of transit repairs and improvements, including modernizing subway signals and stations and expanding the electric bus fleet.

The plan has been politically contentious with many Republicans, and some Democrats, calling it another tax on drivers. President-elect Donald J. Trump has vowed to kill it when he takes office this month, saying it would drive visitors and businesses from Manhattan.

At least 10 lawsuits have been filed seeking to keep congestion pricing from taking effect. The plaintiffs span an array of opponents, including Vito J. Fossella, the Staten Island borough president, Michael Mulgrew, the president of the United Federation of Teachers, and the Trucking Association of New York, a trade group representing delivery companies.

The latest legal challenge to the program came Friday when New Jersey officials sought a last-minute injunction based on what they said was congestion pricing’s potential environmental impact on their state. The judge, who last week ordered federal transportation officials to review and explain some aspects of the program, denied the motion.

It will most likely be unclear for some time whether the tolls significantly reduce traffic and by how much. Lowering them will probably deter fewer drivers.

State officials said the original plan was expected to reduce the number of vehicles in the congestion zone by roughly 17 percent. They have not specified how the scaled-back program will compare except to say they expect it to cut traffic by at least 10 percent.

Mr. Day, of Rockland County, and other opponents have criticized the toll prices, saying that drivers will pay the same rates no matter how much time they spend in the congestion zone, or how much they drive around inside it and contribute to congestion.

Last week, Ms. Hochul ruled out a surge pricing option that would have allowed for a 25 percent surcharge on heavy traffic days.

The tolling plan also does not directly charge drivers and owners of for-hire vehicles, which have exploded on city streets since Uber’s arrival in 2011. Instead, a small per-trip fee — $1.50 for Ubers and Lyfts; 75 cents for taxis — will be added to each fare and paid by passengers.

Many supporters believe the tolling program will create an important long-term revenue stream for transit improvements.

“Congestion pricing is a very good way of raising money for the M.T.A.,” said Rachael Fauss, a senior policy adviser for Reinvent Albany, a government watchdog group. “It’s a revenue source that isn’t tied to ridership. This is exactly the type of financing you want because it’s a stable, proven revenue source.”

Opponents counter that the M.T.A. should find better ways to spend the money it already has. The critics fault the authority for costly operations and spending on projects that routinely go over budget.

M.T.A. officials have said they have improved efficiency in recent years, including on some of their biggest projects, like an expansion of the Long Island Rail Road in 2022 that was completed $100 million under budget.

Now, with hours to go before the tolling program becomes reality, both sides of the congestion pricing divide are getting ready. Some supporters planned to gather early Sunday at a tolling site along 60th Street to mark the official start of the program.

Mr. Schwartz will not be there. After decades of calling for congestion pricing, he was not expecting it to finally happen while he was away on vacation in Aruba for the holidays.

On Friday afternoon, Mr. Schwartz emailed: “I’ve got my bottle of champagne on ice!”

Wesley Parnell contributed reporting.