Sixteen of the 20 largest U.S. cities are experiencing growth rates faster than their surrounding suburbs, reversing a trend towards suburban sprawl that has held since the '20s. Retiring baby boomers and millenials alike are relocating to the city in search of culture, walkability and faster-paced lifestyles. For banks, this transition is contemporaneous with hard economic times. The proclivity of younger city dwellers toward online banking has dovetailed nicely with many banks' impulse toward downsizing branches to cut costs. But regulators worry the next lending bubble could appear in the urban real estate market. Bankers are also wary, looking for signs of overcapacity after getting burned in the housing bubble.