They argue that it can lessen traffic and reduce emissions from cars.

Now there's a new reason to be gung-ho about public transit -- it helps make people skinnier. That's according to a study published in the August issue of the American Journal of Preventive Medicine.

City planners and citizens alike frequently push for better public transportation.

Researchers surveyed communities in Charlotte, North Carolina.

One survey was done before the city had finished building a new light rail system, and the second was done from 12 to 18 months after the rail's completion.

Turns out that people who started using light rail to commute had a significant average weight loss over that time -- equivalent to about six and a half pounds for a five-foot-five person.

They asked about the residents' level of physical activity, body mass and use of public transportation before and after the light rail.

The users were also 81 percent less likely to become obese over the time scale surveyed.

The researchers say that an environment based on mass transit influences health, because rail users are get walking to and from stops.

And since illness is costly, here's another way public transit saves money. exercise