PITTSBURGH -- When the United States declined to ratify an international treaty that would impose limits on emissions of carbon dioxide and other gases, four twentysomething Pittsburghers decided to take action.
They formed Car Free Pittsburgh, which promotes walking, bicycling, and public transportation. The group is part of the World Carfree Network, a loose-knit coalition of more than 40 groups that believe cars hurt the environment, the economy, and society.
The car-free movement is growing in the United States, although more slowly than in other countries. An estimated 100 million people participate in International Car Free Day each Sept. 22 in 1,500 cities worldwide, according to the Sierra Club and other groups.
But it's not an easy sell, especially in an older, hilly city like Pittsburgh, one specialist said.
''I think it would be an extremely difficult, if not miraculous, feat if organizations would be able to sway the general public, let alone the average Pittsburgher, to give up their cars," said Audrey Guskey, a consumer trends specialist at Duquesne University. ''Instead of retreating from technology, we're constantly grasping at it and seeking more of it."
Nonetheless, the local group is asking drivers to pledge to use their cars less, and educating people about the cause.
''In 20 years, I think the goal would be a society that doesn't focus as much attention on and rely as heavily on the car," said Katie Bombico, one of Car Free Pittsburgh's founders. ''Any small step is going to be a positive one."
Although the Pittsburgh group's founders bicycle or walk as much as possible, Bombico does drive. She said she uses her car to drive to work and to visit her parents, who live five hours away, but otherwise stays out of it.
''We realize, for many people, you just can't jump into a car-free life," Bombico said.
Steve Schmitt, director of the Bethlehem-based Coalition for Appropriate Transportation, said people aren't motivated by the message that cars are responsible for ills ranging from obesity to suburban sprawl.
''When I first started out 15 or 16 years ago, I was very upset. I thought that if I explained all these things to people that they would stop driving," Schmitt said. ''I soon discovered people were too addicted and too dependent on their cars."
The group tried to get folks to stop driving 1 day in 5 and then one day a month, but both efforts failed. Schmitt said that's when the group focused on educating about transportation alternatives.
The World Carfree Network, based in Prague, publishes ''Car Busters" magazine and organized its fifth annual world conference this summer in Budapest.
''It's about people moving around by walking, by cycling, by public transportation," said Randall Ghent, codirector of the network's International Coordination Center. ''Obviously, [the car] is not going to disappear overnight. But we strive for a society where its convenience is lowered and the convenience of these alternatives is raised."
Some cities -- including Montreal, Seattle, San Francisco, and Chapel Hill, N.C. -- participate in International Car Free Day or sponsor their own, urging residents not to drive.
''There's no guilt. We're not like that. We're not hardcore," Chapel Hill's mayor, Kevin Foy, said. ''But what it is supposed to do is expand your mind that you can do other things, and it can be very pleasant to move around in other ways."
Foy said Chapel Hill, neighboring Carrboro, and the University of North Carolina invested in that idea.
They operate a combined transit system with an annual budget of more than $12 million, serving about 6 million bus riders a year. The towns' combined population is about 75,000, plus some 25,000 UNC students.
Ridership has doubled since the transit agency decided three years ago to make the service free. The cities recoup lost fares by not having to widen roads, build parking areas, and other car-related infrastructure, Foy said.