One out of five older Americans can’t drive. And most of these non-driving senior citizens stay home on most days. This leads to obvious health risks: sedentary lifestyles and social isolation are both associated with higher rates of disease and earlier death.

The interesting thing, though, is that in denser areas—cities and suburbs with at least 15 people per acre—most senior citizens get out of the house every day, even those who can’t drive. That suggests that, as the population ages, creating dense, pedestrian-friendly neighborhoods that give seniors greater mobility may be a significant public health priority. (Neil Peirce wrote a good column on this issue in the wake of last summer’s elderly driver crashes.)

Update: According to the report linked above (which was co-released by the Surface Transportation Policy Project and the American Association of Retired Persons), there are about 44,300 residents of greater Portland, and 77,600 residents of greater Seattle, who are over 65 and are unable to drive.