These mayors have added quality public spaces and green zones, increased recreation areas for young people, reduced traffic congestion and improved air quality - thereby upgrading the quality of urban life, and attracting business investors. These mayors recognize that by reducing greenhouse gases they can improve the daily lives of their residents and improve planetary health at the same time.

In London, Mayor Ken Livingstone implemented a congestion charge that cut traffic gridlock in the central business district by 30 percent. Bus ridership zoomed 40 percent, public spaces in Trafalgar Square were reclaimed for pedestrians and cyclists and air pollution that threatened health and climate dropped almost 20 percent. Business leaders and civic organizations hailed the success.

On Jan. 3, Stockholm followed suit with its own congestion charging system that reduced traffic by 16 percent. More than 15 cities in England are considering similar steps.

In Seoul, South Korea, Mayor Myung-Bak Lee transformed 50 miles of traffic-choked city streets by introducing high-speed bus lanes. He tore down a 6-kilometer elevated highway and replaced it with a new riverfront park. These efforts are so popular that 84 additional elevated roadways have been slated for demolition. This January, several major environmental organizations will honor Mayor Lee with a Sustainable Transport Award in recognition of his spectacular achievements. Even more important, Mayor Lee's leadership has been so well received by the electorate that he has become a leading presidential candidate.

In Bogota, Colombia, Mayor Enrique Penalosa rejected aid agencies' advice to build more highways and instead built the world's highest quality rapid bus transit system and hundreds of miles of new walking and cycling paths. A traffic-snarled metropolis has been transformed into one of Latin America's most beautiful cities. Penalosa is now a presidential contender.

In the United States, mayors and governors are often taking environmental initiatives in advance of federal policy. Cleveland, Los Angeles, Pittsburgh and Honolulu all have impressive new rapid bus rapid transit systems. Chicago has added hundreds of miles of high-quality bike lanes. Milwaukee and San Francisco have torn down decrepit urban motorways.

Civic and government leaders from New York to San Francisco are looking to build on the successes of London with traffic congestion charges. States like California and New York - which together account for one-third of the country's tailpipe greenhouse gas emissions - have adopted clean vehicle programs that will cut climate and diesel pollution faster than federal rules.

For the first time in history, more people live in urban environments than in any other human habitat, from the booming mega-cities of the developing world to the sprawling metropolises of the United States. Urban planning will have a major impact on the health and quality of life of billions of people. It will dictate how much time city residents and suburban commuters spend in gridlock, whether the sharp rise in childhood asthma will continue, how many children will die or be disabled due to toxic exposures, the death rate among older urban dwellers due to respiratory diseases, and whether city dwellers will have any exposure to natural spaces.

Enhancing urban viability also helps reduce suburban sprawl, ultimately reducing the greenhouse gases that foul our atmosphere and cause global warming.

Recent developments in London, Bogota, Seoul and elsewhere demonstrate that when leaders have the political vision to manage traffic growth and improve basic amenities like parks, clean transportation and healthy air, city life dramatically improves, and the entire world reaps the benefit. By acting now to transform urban communities, mayors have seized the initiative from national leaders who seem to lack the will to free themselves from the myopia of short-term interests in order to promote an agenda of environmental responsibility.

Michael Replogle is transportation director of Environmental Defense, 1875 Connecticut Avenue NW, Washington D.C. 20009. Walter Hook is executive director of the Institute for Transportation and Development Policy, 127 West 26th Street, Suite 1002, New York, N.Y. 10001. Distributed by Knight Ridder/Tribune News Service.