Someday soon, drivers entering downtown Manhattan can expect to pay for the pollution and traffic jams they cause.
Congestion pricing is a way that places such as Stockholm and Singapore are trying to unclog streets and clean up their air by making it more expensive for drivers to bring dirty vehicles into town.
With traffic bringing many cities to a standstill, air pollution killing an estimated 4 million people per year, and concerns about climate change growing, interest in finding ways to clean up transportation is increasing worldwide.
Economists love congestion pricing. Drivers? Not so much.
But voters in cities that have tried it have come to accept it.
The policy typically works by drawing a border around a city's downtown business district and charging vehicles to cross the border. Some cities have gone beyond congestion charges and impose extra fees based on the vehicle model's pollution levels.
London keeps track of vehicles with a network of cameras that photograph license plates. In other cities, cars carry electronic tags. Some cities, rather than identifying individual vehicles, simply bar vehicles on certain days based on license plate numbers.
Free roads aren't free
New York City has begun holding public meetings to work out its congestion pricing plan, the first in the United States.
Under current proposals, drivers would pay between $9 and $23 to drive passenger vehicles south of Central Park, with some exceptions.
The money raised would go toward improving the city's public transit system.
The idea behind congestion pricing is to make people pay for something that they generally think of as free but isn't, said Williams College economist Matthew Gibson.
"When I decide to travel a mile on an unpriced public road, I'm not thinking about the cost I'm imposing on other members of society in the form of accident risk, air pollution and congestion," he said.
Congestion pricing imposes that cost. If the cost is high enough, drivers will look for alternatives such as public transportation, carpooling, biking or walking.
Studies have found that congestion pricing does work for the most part. But it needs to evolve.
For example, in 2008, Milan started charging high-pollution vehicles a fee to enter the city's central business district. It worked. Traffic cleared up — for a while.
Drivers did what the policy intended for them to do: They replaced their old, dirty vehicles with newer, cleaner ones. And they hit the roads again. Traffic came back.
So, in 2012, the city imposed a congestion fee on all vehicles.
A glimpse at how effective the policy was came when an Italian court put it on hold temporarily in the middle of 2012.
Traffic spiked immediately.
Researchers found that the congestion fee was reducing traffic by 14.5% and lowering air pollution between 6% and 17% — a big drop, considering the pollution fee had already cleaned up vehicle emissions.
Congestion and pollution fees don't always do much to clear the air, experts say. Sometimes other pollution sources, such as coal-fired power plants or heavy industries, cause more pollution than vehicles, for example. And sometimes other measures, such as increasing vehicle efficiency standards, may make the impact of the fees less obvious.
Winning over voters
What is obvious, studies have found, is how congestion and pollution fees clear the roads.
In Milan, for example, "the immediate result was the reduction of traffic congestion," said Bocconi University economist Edoardo Croci. "It is an immediate and evident impact that people notice."
That impact has persuaded voters to keep these policies, even though most were opposed to them at first.
Milan's pollution fee was not popular when officials proposed it. But voters agreed to expand the fee to all vehicles in 2012 after they saw how the pollution fee had cleared the streets.
The same thing happened in Stockholm. Solid majorities opposed a congestion fee when the city launched a six-month pilot program in 2006. But voters approved it permanently after the pilot ended.
"The initial opposition was only because of the fear of something new," Croci said. "But once the advantages were evident, most people were in favor of the charge."
Both cities invested heavily in public transit before the fees kicked in.
That's critical, experts say. The policy won't work if people don't have another option besides driving.
A hard sell in U.S.?
While New York City has an extensive public transit system, congestion pricing "might be a much harder pitch to make for other large U.S. cities," said economics Ph.D. candidate Matt Tarduno at the University of California, Berkeley.
In sprawling cities such as Los Angeles or Phoenix, he said, "people would say, 'Well, I don't want to pay this toll, and if I don't pay the toll and can't drive, what else am I going to do?'"
Without good alternatives, congestion fees can hit the poor disproportionately. Critics note that rich people can afford to drive polluting cars downtown if they want.
New York City plans to exempt people earning less than $60,000 per year.
It's a balancing act, Tarduno said. Lower-income drivers tend to drive older and less efficient cars, which can make the policy less effective.
New York is planning a lengthy public review process, followed by months more to roll out the program. It may be another two years before Manhattan drivers start paying for their pollution and congestion.