Prospect Park is closed to cars on weekends, and on weekdays its West Drive is only open to cars from 5 to 7 in the evening, and its East Drive from 7 to 9 in the morning, and 5 to 7 in the evening.Furthermore, Transportation Alternatives, the well-known cyclists' advocacy group, is pressing -- as it has for many years -- for a "car-free summer."The trend is also found in Central Park in Manhattan, and throughout the country. For example, in San Francisco's Golden Gate Park, every Saturday from late May through September, bicyclists, joggers and pedestrians will have free rein on almost a mile of John F. Kennedy Drive the main drag through the park. Vehicles are already prohibited in parts of the park on Sundays.And there have been dozens of similar moves in at least 20 American cities in the past three years. "Cities across America are increasingly declaring that parks are for people, not cars ... and closing roads within parks is one result of that," says Ben Welle with the Trust for Public Land's Center for City Park Excellence in Washington, D.C.In Prospect Park, the closing of the West Drive during the morning rush hour was a victory won by Transportation Alternatives last year, but the organization doesn't think that's enough."The evening rush hours are a peak time for joggers, cyclists and other people using the park," says Paul Steely White, the group's executive director, who adds that few vehicles used the West Drive during the morning anyway.One reason the group wants a car-free summer is "to show that if we close all the drives to cars, the traffic situation won't descend into chaos," says White. At some public meetings, residents of nearby Brooklyn neighborhoods have said they fear that if the drives are closed, motorists will clog nearby streets with traffic during the rush hours.Studies nationwide, however, are showing that traffic problems can be minimized, while shops and museums get more visitors and residents begin to cherish their where-the-action-is location.Not everyone is convinced, saying the jury is still out on how no-car zones affect neighborhood vitality. In San Francisco, for instance, the de Young Museum has said its delivery schedule must be adjusted because of the new road closure, and it is concerned that patrons with physical disabilities may not be able to get to the museum as readily.In Prospect Park, the hours that cars are permitted in the park have decreased since the city first declared "car-free Sundays" in the 1970s. The restrictions on weekday non-rush-hour use came in 2003.Smaller U.S. cities, from Davenport, Iowa, to Huntington Beach, Calif., are also starting to create car-free zones, according to Welle's studies. Beginning this month, El Paso will detour cars from seven roads every Sunday from 7 to 11 a.m. so that cyclists, joggers and pedestrians can use them instead. "Climate change and the obesity crisis have [rejuvenated] the movement for car-free space," says White of Transportation Alternatives. As of last year, he notes, more of Earth's inhabitants live in cities than in rural areas. "Now we have to figure out what urban habitat will sustain ourselves ... it's all about reducing car use."