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The Washington regional population is booming—-but more people doesn’t mean more driving, according to a report out today from the National Capital Region Transportation Planning Board.

The region’s population grew by nearly 350,000 people, or 7.3 percent, between 2005 and 2011, the report states, but total daily driving hardly budged, holding firm right around 110 million vehicle-miles per day. In other words, the average number of daily miles driven by residents of the region fell from 22.9 in 2005 to 21.5 in 2011.

Where in the region did this drop occur? Well, everywhere. But the biggest decline came in the outer suburbs of Frederick, Charles, Loudoun, and Prince William Counties, where the drop was more than 10 percent. In the “regional core” of D.C., Arlington, and Alexandria, per-capita driving declined 5.6 percent, to 15.1 miles per day.

That the biggest decline in driving is occurring in the suburbs might seem surprising, given that these areas are largely car-dependent. But a Transportation Planning Board study last month found that the commutes that saw the biggest gains between 2000 and 2011 were Montgomery County to Montgomery County (+42,000), D.C. to D.C. (41,000), Loudoun County to Loudoun County (31,000), and Prince William County to Prince William County (29,000). In other words, many more suburban residents are taking shorter commutes within their own counties, rather than taking long drives into the District, and that—-combined with the fact that transit usage has increased in every jurisdiction in the region—-could account for the drop in average mileage covered by suburbanites.

Transit usage is still minimal in the outer suburbs, though: Among Loudoun County residents, for example, it comprises just 2.1 percent of commutes. But in D.C., it’s another story: Transit commute mode share has jumped from 32.3 percent in 2000 to 40.2 percent in 2011. Add to that the growing popularity of Capital Bikeshare and the fact that more D.C. workers are choosing to live closer to their jobs, and D.C.’s drop in driving makes plenty of sense.