More than 220 million vehicles of all descriptions now jostle for space on America's roads, but they have at least one thing in common. Whether they're Mini Coopers or Hummers, they all take up parking spaces. Preferably free ones.

But perhaps not much longer.

It has come to seem as basic an American right as free speech and freedom of religion, but free parking is facing increasing scrutiny. By eliminating free spaces and raising parking rates in downtown business districts, several cities are trying to reduce congestion caused by "cruising," or circling repeatedly around several blocks in a quest for a free spot that results in wasted gas, increased pollution and a greater risk of accidents.

"It's probably where the most fat is," said Wiley Norvell of the New York non-profit group Transportation Alternatives, referring to cruising. "These aren't people who are bound for destinations. They're at their destination."

In a study this year, the group surveyed drivers stopped at red lights in the popular Park Slope neighborhood of Brooklyn. As many as 45 percent of those polled were looking for parking spaces. A similar study in Manhattan's SoHo district last year found that 28 percent of the drivers were trying to find a free spot.

To tackle its parking problem, Redwood City, Calif., introduced new high-tech parking meters in its downtown district in March. The meters allow the city to charge variable rates, ranging from 25 cents to 75 cents an hour along the busiest shopping street at peak times.

Bus ridership in a Portland, Ore., shopping district has tripled in 10 years, thanks in part to 75-cent-an-hour charges at previously free parking spots.

And San Francisco is experimenting with a system that free-market economist Adam Smith would have loved. By tracking usage and availability of parking spaces, the system can adjust the price of parking based on demand.

Taking aim at free parking isn't the only approach to reducing congestion. New York Mayor Michael Bloomberg proposed recently that drivers pay an $8 daily fee to enter much of Manhattan from 6 a.m. to 6 p.m. on weekdays. The fee for trucks would be $21. Revenue from the charge -- estimated at a net of $400 million a year -- would help pay for mass transit projects.

The plan, based on central London's congestion charge of $16 a day, would have to be approved by the state legislature. It has stirred up opposition from politicians whose constituents commute to jobs in Manhattan, as well as parking garage operators who fear their business would suffer.

In London, parking isn't cheap

In an Earth Day speech proposing initiatives aimed at making New York "the first environmentally sustainable 21st Century city," Bloomberg acknowledged that the congestion charge was the most controversial.

But he said London had shown that the charge works. London's plan, which was expanded recently, has reduced daily traffic in the central downtown area by 20 percent and auto emissions by about 15 percent.

One drawback of Bloomberg's proposal is that it does not address the problem of cruising for parking, said professor Donald Shoup, a parking expert at the University of California, Los Angeles.

"It's very sensible to think of a congestion fee, but not if the people who drive into Manhattan think they're going to be able to park free at the curb," said Shoup, author of "The High Cost of Free Parking." "A lot of the congestion you see in New York is people hunting for a curb space."

In London, metered spots run as high as $10 an hour.

Shoup says cities should adjust meter prices to achieve an 85 percent occupancy rate for curb spots, a target that Redwood City, Portland and Anchorage have adopted. That rate would mean that drivers would not have to circle the block repeatedly to find a spot, but enough spaces would be occupied to maintain revenues.

Where revenues could go

Merchants often complain that high-priced parking hurts their businesses, driving shoppers to malls with free parking. But Shoup said officials in Pasadena, Calif., were able to persuade merchants in the Old Pasadena district to agree to meters as long as the revenue was used to pay for improvements in the formerly dilapidated shopping area.

A similar idea is under consideration in Chicago for the 53rd Street business district on the South Side and on Milwaukee Avenue in Logan Square. The City Council passed a resolution in September calling for the creation of "transportation enhancement districts" in those areas.

Within the districts, the current meter rates, generally 25 cents an hour, would be raised to $1 an hour. The city still would get the revenue from the old rate, and it would split the 75-cent increase with the neighborhood transportation districts. The neighborhoods would use the money to help pay for sidewalk improvements, beautification and cleaning.

"We're just trapped in an antique way of thinking," said Shoup, referring to free parking. "If you believe in global warming, and you say, 'I'll drive for an hour to find free street parking,' there's a disconnect there."

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NYC plan would blaze bike trails

When New York Mayor Michael Bloomberg announced his plan to add a congestion charge on vehicles, he also proposed increasing the number of bicycle paths in the city.

His plan also would require commercial buildings to have indoor parking facilities for bikes.

But in the U.S., bike commuters face more challenges than their peers in European cities, including strong opposition from some small businesses, car owners and parking garage owners who balk at proposals to limit parking, shrink driving lanes or reduce speed limits.