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 a 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 nonprofit 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 Park Slope neighborhood of Brooklyn. As many as 45% of those polled were looking for parking spaces. A similar study in Manhattan's SoHo district last year found that 28% of the drivers were trying to find a free spot.To tackle its parking problem, Redwood City, Calif., introduced high-tech parking meters in its downtown district in March. The meters let the city charge variable rates, from as little as 25 cents an hour to as much as 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.Paying to commuteTaking aim at free parking isn't the only approach to reducing congestion. New York Mayor Michael Bloomberg proposed recently that drivers pay an $8 fee to enter Manhattan from 6 a.m. to 6 p.m. weekdays. The fee for trucks would be $21. Revenue from the charge -- estimated at $400 million a year -- would help pay for mass transit projects.The plan, based on London's congestion charge of $16 a day, would have to be approved by the state Legislature and 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 an Earth Day speech proposing initiatives aimed at making New York "the first environmentally sustainable 21st-Century city," Bloomberg acknowledged the congestion charge was the most controversial, calling it "the elephant in the room."But, he said, London has shown that the charge works. London's plan, which was expanded recently to cover a bigger section of the central metropolitan area, has reduced daily traffic in the covered area by 20%, and auto emissions are down by about 15%.Adjusting meter pricesOne drawback of Bloomberg's congestion charge proposal is that it doesn't address 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."By contrast, London has some metered spots that run as high as $10 an hour.Shoup says cities should adjust their meter prices to achieve an 85% occupancy rate for curb spots, a target that Redwood City, Portland and Anchorage, Alaska, have adopted. That rate would mean drivers wouldn't have to circle the block repeatedly to find a spot, but enough spaces would be occupied to maintain parking revenues."We want the parking spaces to be well-used but available," Shoup said.Making merchants happyMerchants often complain that high-priced parking hurts their business, driving shoppers to malls with free parking. But Shoup said officials in Pasadena, Calif., persuaded merchants in the Old Pasadena district to agree to meters as long as the revenue was used to pay for improvements such as better lighting and signs 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."