We already know living in a pedestrian-friendly city can make you healthier and happier. New research shows it's good for you bank account, too.

Average gross domestic product per capita—a fancy way of saying approximated average income—in metro areas ranked as "highly walkable" is 38 percent higher than in "low walkable" areas. In dollars and cents, that means the average GDP per capita in places that promote walking is $60,400 compared to $43,900 in those that do not. It's not quite clear why that is, the new study says, but "evidence suggests that encouraging walkable urbanism is a potential strategy for regional economic development."

Foot Traffic Ahead: Ranking Walkable Urbanism in America's Largest Metros was produced by the Center for Real Estate and Urban Analysis at George Washington University School of Business and Smart Growth America. It examined how much retail and office space was concentrated in areas with a Walkscore.com rating of 70 or more (out of 100).

In Washington, D.C., which topped the list, 43 percent of that space—where people live, work, and shop—is in areas where walking is easy and safe. New York scored 38 percent, Boston 36. In Orlando, Phoenix, Tampa, and other cities near the bottom of the list, under 10 percent of that real estate is in walkable areas.

Please don't assume moving to one of those cities—or setting your treadmill to 3 mph—will put more cash in your pockets. The correlation likely can be explained, the report says, by the fact residents of more walkable cities tend to be better educated than those in less pedestrian-friendly areas. Get more education, make more money.

This research doesn't indicate whether more educated people make cities more walkable or walkable cities attract the better educated, or whether both factors are at play, says Chris Leinberger, lead author of the report and president of the Center for Real Estate and Urban Analysis. While there's no clear causal connection behind this "very substantial correlation," he says, cities shouldn't wait to make life better for pedestrians. Whatever the reason for the correlation, it's clear that more walkable areas are wealthier areas. That should be reason enough to slow traffic, add crosswalks and bike lanes, and improve public transit.

This chart from the report shows how GDP and rates of higher education drop as pedestrians become less pampered: