hen Queen Elizabeth opened the new City Hall in London last year, some observers compared the building, designed by the architect Norman Foster, to a giant eye. And that is exactly what it looks like -- a glassy postmodern eyeball on the south bank of the Thames, staring across the river at its staid Georgian and Victorian neighbors as if to say, ''Welcome to the 21st century.'' Atop the building is a semicircular penthouse called London's Living Room, walled with windows that offer a commanding view of the city below. The idea behind this room is to offer a place for Londoners to gather, if only through the medium of a television camera, for the kinds of serious, big-family sit-downs that go along with governing a sometimes dysfunctional city of more than seven million -- a city so decentralized, in fact, that until three years ago it never had an elected mayor.

On an unusually bright morning earlier this year, that mayor, Ken Livingstone, strides into the room before a bank of cameras, and with an unusually pleased look on his dour face, announces a coup, one that has eluded dozens of large cities like New York, Los Angeles and Paris. He has not conquered crime or poverty, but he may very well have hobbled an urban enemy seemingly just as invincible: the car. Livingstone has just begun the world's most radical experiment in reclaiming the city from the tyranny of the automobile, a power struggle that cities have been losing in humiliating fashion for more than half a century. Since well before his election, he has been warning Londoners that far too many of them (about 250,000 a day) are trying to drive into far too small a place -- central London -- polluting the air, choking commerce, slowly strangling their own livelihood. To stop them, the mayor decided to draw a line, literally.

The line formed a lopsided oval around eight square miles of the historic inner city. Almost anyone who drove across the line during business hours -- in fact, almost anyone who moved or even parked a car on the street within it after Feb. 17 -- instantly owed the city of London $:5 (about $8) a day for every day it happened. If a driver failed to pay, one of more than 700 vulturelike video cameras perched throughout the zone would capture his license plate number and relay it to a computer, leading to a huge fine. And if the driver declined to pay those fines? The mayor vowed, only half-jokingly, that the city would relentlessly track his car down, clamp it, tow it away and crush it -- ''with or without the driver inside.'' Few would be exempt, not even volunteer social workers, teachers, foreign diplomats or undercover police officers.

The idea behind his assault on automotive freedom was neither new nor very hard to understand. If a finite resource is free, human beings tend to use it all up, regardless of the consequences. If it has a cost, they tend to use it more rationally. Livingstone, a far-left Socialist, won his mayoralty largely on the promise of applying this tough-love theory to London's streets. But in the weeks just before the ''congestion charge'' began, it sometimes seemed that he was the only one who believed it would work. The newspapers were full of derisive nicknames for it, like ''Ken-gestion'' and ''Carmaggedon.'' Samantha Bond, the actress who plays Miss Moneypenny in the most recent James Bond movies, became the sympathetic face of the opposition, presiding over a protest with hints of civil disobedience at the West End theater where -- somehow fittingly -- ''Les Miserables'' was being staged. Tony Blair's government, which had given London and other British cities permission to levy such traffic charges in the first place, carefully distanced itself from the plan. And the bookmaking firm William Hill, one of London's most able arbiters of public sentiment, began offering 4-to-1 odds that it would fail by the end of the year. (The odds that Livingstone would be out of office before the end of his term were put at 10 to 1.)

On this sunny Tuesday morning, however, it appears that the mayor has beaten at least the first of those odds. The number of cars entering the cordon zone the day before, the first day of the charge, dropped by about 60,000, remarkable even in the context of a school holiday. One automobile group estimated that average speeds in central London had doubled, nothing less than a miracle in the world of road policy. Livingstone, addressing his public in a droopy suit, bright blue tie and a pair of sensible thick-soled walking shoes, declares it ''the best day we've had in traffic flow in living memory'' and reports that he has even taken a call from the government's transport minister, John Spellar, a Labor Party archenemy who had helped to expel Livingstone from the party three years earlier when he launched his renegade mayoral bid.

Livingstone's eyes twinkle as he relates the conversation. ''He said, 'Clearly the devil looks after his own,' and we had a good laugh,'' the mayor says.

When a reporter asks whether the mayor has truly considered the consequences of the scheme failing, especially with his re-election campaign only a year away, Livingstone's nasal Cockney voice, already as affectless as a door buzzer, drops to a full deadpan. ''I never consider my own future when making political decisions,'' he says. He pauses for effect. ''How can you be so cynical?''

As television crews troop out to the balcony to shoot the light traffic wheeling around the Tower of London, a good laugh is had all around the living room.

he exchange, however, goes straight to the heart of cities' tangled history with the automobile -- undoubtedly the most inefficient, and most aggressively defended, means ever conceived for transporting large numbers of people through crowded places. The idea of using a price tag to regulate driving into crowded places has been around for years, but its progress has been slowed by two problems, one big, the other gigantic. The first was simply technical: how would you charge for entry into entire cities or neighborhoods without putting tollbooths everywhere and causing more congestion? That obstacle has now been largely overcome with high-speed electronic tolls, sharpshooter cameras (originally developed for antiterrorism purposes in London) and even the development of satellite tracking of cars.

The gigantic problem is political. Since at least the end of World War II, the battle between cars and cities, a battle over the shape of the city itself, has been an epic mismatch. An oversimplified chronology would read something like this: the car helps to create sprawl; sprawl siphons people and political power away from the hearts of cities; the car returns to attack the city, which was never designed to accommodate so many; the city is forced to transform itself, ceding sidewalks to streets, trolley tracks to traffic lanes, parks to parking lots, whole neighborhoods to expressways.

In the United States, the critic Lewis Mumford foresaw a grim end to the whole process: ''a tomb of concrete roads and ramps covering the dead corpse of a city.'' While the effects have not been quite that dire yet, the imbalance remains tremendous. On a purely human level, it can be witnessed any weekday in Times Square, where armies of angry pedestrians crowd around S.U.V.'s pinioned in crosswalks, the drivers inside easily outnumbered 100 to 1.

But those drivers and the people who profit from them in cities -- principally garage owners, automobile clubs and road builders -- have had tremendous political influence over the years. They have portrayed unfettered access to public tax-supported roads as something like a modern amendment to the rights of man. And while it may be in the long-term interests of drivers to pay for using some roads in order to make them passable again, to put that money into subsidizing more efficient conveyances like trains and buses, city leaders have long viewed administering that corrective as something close to electoral suicide. Even the most crusading anti-car mayors -- like John V. Lindsay in New York, who came within weeks of ordering a Midtown traffic ban in the early 1970's, and Edward I. Koch after him, who came almost as close to imposing tolls on the free East River bridges -- have ultimately backed down or lost their battles.

Though it might seem like a relatively new phenomenon, saturation traffic has existed in many cities for decades, virtually unchanged. Depending on whom you believe, it is incredibly destructive, costing London alone over $300 million a year in lost productivity and revenue just because of congestion in the tiny central portion of the city. (One New York City study in the late 1990's found that traffic problems in Manhattan cost the city as much as $4 billion a year in lost productivity.)

With its mazelike medieval streets, London was a city plagued with congestion long before the car. In his diaries, Samuel Pepys twice recorded being stuck in 17th-century horse-and-buggy jams. When the car came along, the original notion was that such age-old transportation problems could be solved if enough new roads were built to handle cities' needs, a strategy called ''predict and provide.'' But by the 1960's, only a half-century after the car came into common use, economists and traffic planners were starting to notice that new roads seemed only to create more traffic.

By 1977, when the British punk band the Jam recorded ''London Traffic'' (''No one knows the answer/No one seems to care/Take a look at our city/Take the traffic elsewhere''), the average speed of a car in central London was 12 miles an hour, or a little faster than the top running speed of a domestic pig. At the turn of the millennium, more than two decades later, many Londoners could only look back on those congested years with nostalgia. The average speed had dropped to less than nine miles per hour for the first time in modern record-keeping, meaning that car travel through Britain's capital was generally as slow as by coach a century ago.

''We're addicted, really,'' Bev Ramsden, a veteran taxi driver and dispatcher, told me one wet weekday morning, inching down the A4 highway through the gray margins of Hammersmith, nowhere near the most congested part of the city. ''Like addicts, I think we're getting to the point where we're realizing how crazy this is. Someone's got to do something.''

It will probably go down as one of the stranger chapters in the history of traffic policy that the man who finally did something is a former lefty radical (once known as Red Ken) applying conservative free-market ideas. In a way, of course, it all makes complete sense: the congestion charge is classic Robin Hood socialism, taking from the comfortable Londoner commuting by Bentley and giving to the commoner hanging from the strap of a packed double-decker bus. But don't misunderstand. While he is a crusader, Livingstone is also a famously foot-sure career politician as interested as any in re-election. Despite his quip for the television cameras, he did not launch his assault without making a lot of practical calculations about its effect on his future. That morning, in fact, waiting downstairs for him in a cavernous boardroom was a group of strategists who were highly paid to do just that. It was telling that most of these strategists were not from London at all but from a place with much worse traffic problems and a much more treacherous political climate for trying to solve them: New York City. (Average traffic speed: about seven miles per hour, no faster than a running possum.)

Only a few months after his election in the summer of 2000, Livingstone began courting Robert R. Kiley, a former C.I.A. official, business leader and transit expert, who as head of the Metropolitan Transportation Authority in New York in the 1980's was credited with resurrecting the city's graffiti-scarred subway system, now considered one of the best in the world. Kiley, given the new title of London's transport commissioner, brought with him another former top New York transit official, Jay Walder, who had become an expert on road pricing at Harvard and in Singapore, where a smaller but much more costly congestion-charging system in place for more than 25 years has cut car ownership to 1 in 10 city residents.

When Kiley arrived in London, most of the attention focused on his transit credentials and how he would use them to rescue the ailing London Underground, an effort in which he and Livingstone, fighting Blair's government, have been largely unsuccessful. But Kiley told me later that he was equally interested in coming to London because of Livingstone's determination to try to right the relationship between the city and the car. If it worked, Kiley knew, it would be seen as a model around the world, and especially back in New York, where more than 250,000 vehicles crowd into the 8.5-square-mile heart of Manhattan in three hours every morning, roughly the same number that enter the eight square miles of central London over the course of an entire workday.

As the leader of a business alliance in the 1990's, Kiley advocated road pricing for Manhattan, but he received no support from Mayor Rudolph W. Giuliani, whose voting base in Queens and Staten Island practically lived in their cars. In many ways London was an interesting parallel, more like New York than any other American city in its atypical transportation landscape. In both cities, as packed as the roads can be, more than 80 percent of workers take some form of mass transit into the central city every weekday morning. In London, as in New York, some drivers are poor. But most tend to have money -- enough to generate political pressure to protect their choice. They are also affluent enough, Kiley points out, to be persuaded to spend a little money to save them something much more valuable: their time.

''We knew all along that the motorist advocates and writers for the newspapers and libertarians and people who are really locked into cars would be critical, but I think the majority of Londoners supported congestion-charging right up to opening day,'' Kiley said later in his office, with a poster of the Brooklyn Bridge behind his desk. ''Would I call it a popular measure? Probably not. But I think that Londoners have long since concluded that someone had to take this dragon on.''

Sitting there that day, as the dragon was being cowed on the streets below, Kiley told me that he had spoken at length about fighting it with another very important potential St. George, one in some ways a lot like Livingstone -- a political outsider who takes the subway to work, who strongly supports the idea of road pricing and who views the prerogatives of driving from a much more jaundiced 21st-century perspective. His name was Michael R. Bloomberg, and he was the mayor of New York City.

Though not mentioned in ''The Power Broker,'' Robert A. Caro's biography of the master road builder Robert Moses, one of the more iconic clashes in the long war between the car and the city took place in New York, with Moses playing a role. He and other planners wanted to slice a highway through the middle of Washington Square Park, the heart of Greenwich Village. It is now hard to believe such a plan was ever seriously proposed, but in 1958 it came close to happening.

At the time, photographed defiantly on the City Hall steps with a giant prop key to lock traffic out of the park, a Tammany Hall leader framed a question that was only then starting to be asked in earnest. Would we, he asked, ''plan and develop our cities in accordance with the needs and wishes of the people who live in them or for the convenience of the vehicles which pass through them?'' The highway through the park was eventually scrapped, but in New York that question, until very recently, has been answered almost always in favor of the passing cars. From 1924 to 1965, car lanes into Manhattan grew from 68 to 120, according to one count, while the number of cars on the street went from 390,000 a day in 1946 (considered intolerable at the time) to more than a million by the end of the 1990's. And that is not because travel has been made more efficient. In fact, it has often been the opposite. In 1907, with trolleys and traffic lanes, the Brooklyn Bridge carried 426,000 people a day; now, with space only for cars, it carries far less than half that number and is often jammed. Convoys of trucks rumble down the decaying streets of Chinatown on their way to New Jersey because tolls on the Verrazano Narrows Bridge would cost them an average of $33 per trip to take the highways that are better designed for them.

Many traffic experts see Bloomberg as the last, best chance -- at least for the foreseeable future -- for anything to change. When he was campaigning, he sought the advice of car skeptics like Kiley. Samuel I. Schwartz, an engineer who worked on East River bridge tolls under both Lindsay and Koch, wrote much of Bloomberg's stridently anti-car campaign platform himself. And Schwartz, who coined the quintessential New York warning ''Don't Even THINK of Parking Here,'' is no moderate on the issue. He advocates charging trucks $50 for using Manhattan as a pass-through and, were it technically possible, $25 a minute for people who want to cruise Fifth Avenue during the height of the holiday season. (''They want to see the Rock Center Christmas tree from their car?'' he says. ''If they do, they should pay for that great privilege.'')

After his election, Bloomberg seemed to be moving in that direction. He decided, in the face of mounting attacks by powerful garage owners, to maintain most of an emergency traffic ban that Giuliani started after the Sept. 11 attacks, preventing single-occupant cars from crossing into much of Manhattan during the morning rush. He has ended the age-old tradition of free Sunday parking in many neighborhoods (including his own, the Upper East Side) and banned turns on some busy crosstown streets -- small changes but ones met with shrieks of protest. His transportation commissioner, Iris Weinshall, even went to London last summer to talk to Kiley and Livingstone about the congestion charge.

But there seems to be a growing sense that Bloomberg could end up among the near-miss mayors on any kind of serious traffic reform. In large part, this is because he has already spent a career's worth of political capital by raising property taxes to fix the city's enormous budget gap, for example, and by banning smoking in bars, a move that would probably get Livingstone sacked in London. Bloomberg and his staff are so nervous about traffic issues that they do not like to talk about them even privately anymore. One city official told me of his particular nightmare: trying to write the speech that Bloomberg would deliver when he cut the ribbons on the new Brooklyn Bridge toll plaza: ''What's he going to say? 'Ladies and gentlemen, these things that've been free for decades and decades. I'm the guy who's going to make you pay for them! Thank you for your support!'''

Kiley says he still believes that Bloomberg could sell a congestion charge, especially in a city where so many take mass transit and only half of the people living at the epicenter of the problem even own cars. ''That's not a bad place to start,'' he says, ''when you know that half the people in Manhattan are going to be with you, almost by definition.''

For all the rest, he adds, ''Bloomberg could use the analogy of, well, look what a difference government has actually made to the subway system. Now we've got to take the next step because we have a subway that's working better, a commuter rail system that's in good shape and lots of room on buses. We've got to really start managing road use. That could be his message.''

Would the message work? New York might not be ready to hear it yet, and the messenger might be killed. But inevitably the city will have to listen, and the brave politician who forces it to come to its senses will be heralded as a visionary. ''Fifth Avenue'' has always had a dull ring to it. What about ''Bloomberg Promenade''?