Mass transportation revised the social and economic fabric of the American city in three fundamental ways. It catalyzed physical expansion, it sorted out people and land uses, and it accelerated the inherent instability of urban life. By opening vast areas of unoccupied land for residential expansion, the omnibuses, horse railways, commuter trains, and electric trolleys pulled settled regions outward two to four times more distant from city centers than they were inthe premodern era. In 1850, for example, the borders of Boston lay scarcely two miles from the old business district. By the turn of the century the radius extended ten miles. Now those who could afford it could live far removed from the old city center and still commute there for work, shopping, and entertainment. The new accessibility of land around the periphery of almost every major city sparked an explosion of real estate development and pulled what we now know as urban sprawl. Between 1890 and 1920, for example, some 250000 new residential lots were recorded within the borders of Chicago most of them located in outlying areas. Over the same period, another 550000 were plotted outside the city limits but within the metropolitan area. Anxious to take advantage of the possibilities of commuting, real estate developers added 800000 potential building sits to the Chicago region in just thirty years lots that could have housed five to six million people. With which of the following subjects is the passage mainly concerned?