Mass transportation revised the social and economic fabric of the American city in three fundamental ways. It speeded up physical expansion, it sorted out people and land uses, and it 62 the inherent instability of life. By opening vast areas of unoccupied land for residential expansion, the buses, 63 , commuter trains, and electric trolleys pulled settled regions outward two to four times more from city centers than they were in the pre-modern era. In 1850, for example, the borders of Boston lay 65 two miles from the old business district; by the turn of the century the 66 ex tended ten miles. Now those who could afford it could live far removed from the old city center and still 67 there for work, shopping, and 68 The new accessibility of land around the periphery (外围) of almost every, major city 69 an explosion of real estate development and fueled what we now 70 as sprawl (城市蔓延). Between 1890 and 1920, for example, some 250,000 new 71 lots were recorded within the borders of Chicago, 72 of them located in outlying areas. Over the same period, another 550,000 were plotted outside the city limits 73 within the metropolitan area. Anxious to take 74 of the possibilities of commuting, real estate developers added 800,000 potential building sites to the Chicago region in just thirty years—lots that could have housed five to six million people. Of course, many were never 75 ; there was always a huge surplus of subdivided, but vacant, land around Chicago and other cities. These 76 pre sent a feature of residential expansion 77 to the growth of mass transportation: sprawl was 78 unplanned. It was carried out by thousands of small investors who paid little care to coordinated land use or to future land users. Those who purchased and prepared land for residential purposes, particularly land near or outside city borders 79 transit lines and middle-class inhabitants were anticipated, did so to create demand 80 much as to respond to it. Chicago is a prime example of this 81 . Real estate subdivision there proceeded much faster than population growth.