What can we conclude from this history of sprawl and the campaigns to stop it? It appears that sprawl has been a feature of urban life since time immemorial. In the past when cities reached a certain level of economic maturity and affluence, densities began to decline at the center as many people who had a choice moved farther out to escape the congestion and pollution of the city, reduce costs, or gain more space. This resulted in a corresponding rise in density at the edge as rural land became exurban or suburban. Within the last century this process has been visible in cities throughout the affluent industrialized world, producing a marked flattening and—until very recently—lowering of the density gradient (fig. 1).
This flattening of the density gradient seems to have happened because a more dispersed landscape has afforded many people greater levels of mobility, privacy, and choice than they were able to obtain in the densely settled large cities that were the norm through the end of the nineteenth century. A great many people have concluded from exactly this kind of analysis that sprawl is inevitable and that efforts to stop it are doomed.
However, in any analysis, it also pays to look at what we don't know or can't explain. For example, there is considerable evidence that—at least in the central city and regularly developed suburbs of many fast-growing American urban areas—the longstanding process of decentralization has actually reversed itself. The line on the chart that records density gradient is still flattening but it is no longer falling. Densities are actually rising sharply both at the center and at the edge in places like Los Angeles, Phoenix, or Las Vegas and more slowly elsewhere in the country. New suburban development at the edge of most American cities is now considerably denser than it was in the decades after World War II.