Fewer than half the adults in the United States meet government recommendations for physical activity, and while there are a number of explanations for that, experts say that access to a safe, convenient place to work out is one of them.

A study released Thursday looks at opportunities for physical activity in parks, gyms, forests, schools and clubs, and on trails, golf courses or any of the dozens of other places you might conceivably exercise in 3,114 of the country's 3,141 counties. It shows, its lead author said, that access correlates well with higher amounts of exercise, and lack of access is associated with obesity and other measures of inactivity.

"Where you live, learn, work and play matters to your health," said Anne Roubal, a doctoral student in population health sciences at the University of Wisconsin, who wrote the paper in Preventing Chronic Disease, a publication of the Centers for Disease Control and Prevention. "It’s truly a neighborhood measure."

Overall, the study concluded, 77 percent of the U.S. population has adequate access to exercise opportunities. Perhaps surprisingly, residents of the District of Columbia ranked highest; fully 100 percent of them have access to some place where they can exercise. Maryland was second, with 91 percent. Mississippi, where just 46 percent of residents had easy access to opportunities for physical activity, had the lowest ranking.

California, New Jersey and Rhode Island residents also had better than 90 percent access. Arizona was the only other state with a sub-50 percent rating.

Regionally, people who live in the northeast and west had better access than residents of the southwest or southeast. In 19 counties, no one had access to one of these places; in 51 counties, everyone did. Roubal said the disparities between regions and counties are the most important revelation of the study, and that people in areas with few opportunities need to begin working with their communities to improve it.

Clearly the definition of "access" is critical here. Based on surveys of people's recreational and exercise habits, Roubal's group defined access as living within a half-mile of a park, a mile of a recreational facility in an urban area or three miles of one in a rural area.

The map above gives you a rough idea of relative access to exercise opportunities, but if you want to see a lot of data on how your county fares, click here.