IS IT A PATTERN ?

American urbanization has followed the same demographic pattern found in every urbanizing society for the last two centuries. The word "urbanization" itself refers to an increase in the proportion of national or regional population living in cities. For the first six thousand years of urban life, no society was long able to maintain an urban percentage greater than from 5 to 10 percent. Starting in late eighteenth-century England, however, one nation after another experienced an accelerating shift from rural to urban population. After several generations of rapid urbanization, the process leveled off toward a new equilibrium in which about three-quarters of the population lived in cities and many of the rest pursued city-related activities in smaller towns. The result, when the urban proportion of a population is graphed against time is an s-shaped curve that turns up sharply for perhaps a century and then tapers off.
Urban growth in the United States has clearly followed these three stages of gradual growth, explosive take off, and maturity. First, the era of colonial or pre-modern cities stretched from the seventeenth century to the 1810s. Second, the rise of the industrial city dominated a century of rapid urbanization from 1820 to 1920. Finally, the third era of the modern city has run from 1920 onward. At each stage, the available technologies of communication and transportation shaped the internal patterns of cities and their distribution across the continent.

http://www.faculty.fairfield.edu/faculty/hodgson/Courses/so11/population/urbanization.htm