I believe it's commute modal share, which asks people which method is their usual way of getting to work. For the U.S. as a whole, transit has 5% [1], so it wouldn't surprise me that not many cities break 15%.

[1] 76% drive alone, 11% carpool, 5% take public transportation, 8% other (walk, bike, taxi, work from home). http://factfinder2.census.gov/faces/tableservices/jsf/pages/...