Just outside Washington, on the grounds of an old farm, a new community is taking shape that researchers think is the kind of place that will help solve the nation's growing obesity crisis.

At the King Farm development in Rockville, homes are being built, streets are being paved, sidewalks are being laid, and office buildings, restaurants and stores are being located in ways that experts say should do one seemingly simple but crucial thing: get people to walk more.

A handful of similar communities have been sprouting up slowly across the nation in the first tentative attempts to counter the sprawl of strip malls, cul-de-sacs and subdivisions without sidewalks that force people to drive everywhere, which -- along with junk food and super-sizing -- is believed to be a major reason that Americans are getting so fat.

"We built communities with no sidewalks, and then we wonder why our kids don't walk to school. We live in gated communities where the garage faces the street and there's no connection with the neighbors, and we don't get out and walk. We drive to everything," said James O. Hill, a weight researcher at the University of Colorado Health Sciences Center. "We've created the perfect environment for creating obesity."

So far, many of the "walkable" attributes of new neighborhoods such as King Farm have been unanticipated consequences of decisions that developers made largely to satisfy housing density requirements or to make their projects more marketable. But the nation's obesity crisis has spurred a new movement to purposefully build communities and retrofit existing ones to make it more natural for people to be physically active.

"We're trying to develop an environment that's health-promoting so we can avoid dealing with treating all the illnesses that result from obesity down the line," said Allen Dearry of the National Institute of Environmental Health Sciences at the National Institutes of Heath, who organized a recent conference in Washington to spur more efforts.

Medical researchers, government officials, sociologists, exercise scientists, nutritionists, city planners, architects, transportation experts, developers and even police have been forming unusual collaborations around the country to foster new living and work environments such as King Farm.

The effort is driven by accumulating evidence that the physical environment plays a crucial role. A landmark University of Maryland study last year found that people who live in the most sprawling counties are the most likely to be overweight, and vice versa. And this month, the first study to examine the issue on a neighborhood level showed that people who live where stores and other businesses are within easy walking distance are significantly less likely to be overweight, primarily because they walk more and drive less.

"Having shops and services near where one lives is the best predictor of not being obese," said lead author Lawrence D. Frank of the University of British Columbia, adding that mixing housing and businesses also may make it easier to eat better by offering grocery stores instead of convenience stores and better quality restaurants instead of fast-food outlets.

Another new study out this month found that poor people and minorities tend to have less access to parks, pools and other facilities that make exercise easier.

Several federal agencies, state and local governments, private foundations and community organizations have begun funding projects to encourage walking and physical activity.

"Our built environment scripts our behavior in many, many different and important ways," said Richard E. Killingsworth, an associate professor at the University of North Carolina School of Public Health who directs one of the biggest programs, called Active Living by Design. "It's the driving force for how we incorporate daily physical activity into our lives, which is one of the keys to maintaining a healthy weight."

The projects span the country:

* In Denver, developers are building a massive new neighborhood on the site of the former Stapleton International Airport that features sidewalks, street patterns that enable people to walk from one place to another, open spaces and other attributes that are conducive to walking, bicycling and other outdoor activities.

* In Columbia, Mo., volunteers are organizing "walking school buses" in which parents escort lines of children to school on foot each day instead of having them sit on the bus. Officials are also slowing traffic flow, beefing up policing and improving crosswalks to make walking safer.

* In downtown Cleveland, volunteers are turning two old industrial sites in the low-income Slavic Village neighborhood into a golf course and a park, and transforming abandoned rail lines into trails for walking, cycling and other activities. Police there are also trying to make it safer for kids to walk to school.

* On the Winnebago Reservation in rural Nebraska, tribal leaders are building walkways across a major state highway that bisects the reservation, and constructing a new cluster of homes and businesses in the center of the community to encourage more walking by residents, who suffer from a high rate of diabetes.

* In Norwich, Vt., doctors are writing prescriptions for patients not to take pills -- but to walk, and volunteers are creating new trails to make it easier for people to follow the doctor's orders.

The federal Centers for Disease Control and Prevention in Atlanta, which unlocked and repainted its stairwells and decorated them with art by employees' children to encourage staff to eschew the elevators, is working to implement such changes on a broader scale.

Federal health officials are helping develop a model planning code that communities can adopt, using local zoning, ordinances and tax incentives to require sidewalks and other measures to promote walking.

Guidelines are also being developed to help employers make workplaces more activity-friendly.

"You could build a golf course and tennis courts, but they will reach only a few people. Walking is the most common and readily available physical activity. We put a focus on it because it's so available," said Andrew L. Dannenberg of the CDC's National Center for Environmental Health.

At King Farm, developers are nearly done constructing 3,200 housing units on 450 acres of old farmland. The nascent neighborhood was designed using so-called New Urbanism ideas, which in many ways re-creates the lifestyle that existed in small towns until the 1940s and '50s.

The development consists of a mix of townhouses and single-family homes on streets that are narrower than typical suburban roads and bordered with wide sidewalks. Many homes have front porches. All the garages are in back. Common areas and parks are scattered throughout. The homes are packed closely together around a commercial center that mixes office buildings with a grocery store, restaurants and other businesses -- all of which have their parking lots in the back to encourage foot traffic.

Even so, the design remains far from ideal. The neighborhood is bordered on all sides by busy roadways that require residents to get back in their cars if they want to leave. They have to take a shuttle bus to get safely to the nearby Shady Grove metro station. Nevertheless, residents praise their new environs.

"This is the first time we've ever been able to walk to a Safeway," said Tiffany Berman, 34, who recently moved to the neighborhood with her husband, Lou. "We really don't use the car at all in the neighborhood. It's a really unusual place."

Because King Farm is one of the largest and newest such developments in the nation, researchers chose it to become part of a four-year, $2 million federally funded study aimed at gauging the effects of such designs on the amount of activity that residents get, and on how much they weigh.

Investigators are studying about 2,400 people in 32 neighborhoods -- half in Maryland and half in the Seattle area, collecting detailed information about their habits by conducting surveys and asking them to wear special meters that measure the physical activity they are getting during two-week periods at different times of the year.

"We know that we can educate people and motivate them to exercise and that works okay for a short time, but within six months to a year most people are no longer active," said James F. Sallis, director of the Active Living Research Program at San Diego State University, who is leading the study.

"But with the built environment, if you build it right in the first place, people seem to automatically incorporate activity into their lifestyle, and their weight is controlled as long as they are there. That's why it's so important to get the built environment right -- because we've been doing it wrong for so long. We want to learn what makes an optimal walking environment."

Tiffany Berman lives in Rockville's new King Farm development, one of a handful of communities nationwide built to encourage walking.King Farm resident Tiffany Berman says she walks to the grocery store.