The amount of time people spend in their cars affects their weight more than their income, education, gender and ethnicity, researchers have found.

The more people drive, the more their waistlines expand and the more likely they are to contribute to the nation's obesity epidemic (search), according to a new study.

The survey of 10,500 metro Atlanta residents found that for every extra 30 minutes commuters drove each day, they had a 3 percent greater chance of being obese than their peers who drove less.

The survey also found that people who lived less than a half mile from shops were 7 percent less likely to be obese than their counterparts who had to drive.

"The more driving you do means you're going to weigh more — the more walking means you're going to weigh less," said Lawrence Frank, associate professor at the University of British Columbia (search) who oversaw the study when he worked at Georgia Tech.

About 91 percent of the people surveyed said they didn't walk to destinations. Many spent more than an hour each day in their cars.

The study is one of the first to look at the link between the environment and obesity, said Kelly Brownell, chairman of Yale University's psychology department and director of its Center for Eating and Weight Disorders (search).

"Studies of this type are very important because they show factors in our environment that can either help or hurt our waistline," said Brownell, who was not involved in the study. "These results show that the environment, affecting our physical activity, is quite influential."

In the study, which is expected to appear in the June issue of the American Journal of Preventive Medicine, researchers tracked participants' travel behavior and measured their height and weight from 2000 to 2002.

The study focused on Atlanta, but Frank said the city is not alone.

"Most regions look very similar to Atlanta — anything that's built after World War II is pretty much auto-oriented," he said. "We need to start to look at the way we're designing our communities ... the collective impact of having to drive everywhere is becoming really large."

Suburban, white men typically weighed about 10 pounds more than men who lived in dense urban areas with shops and services, according to the study, which will be presented Thursday at a national obesity conference in Virginia.

The study was paid for by $4 million in grants from the Georgia Department of Transportation, Georgia Regional Transportation Authority and the federal Centers for Disease Control and Prevention. The Atlanta Regional Commission and the Environmental Protection Agency also participated.