Urban Dynamics

What makes cities degenerate? What can be done to revitalize stagnant urban areas? Are present urban management programs effective, or do they deepen the difficulties in our cities? This study offers some surprising answers to these critical questions and will undoubtedly spark spirited, but fruitful, debate. Professor Forrester develops in detail a dynamic model, or theory, of the processes that affect the life and decay of cities. Such models, permitting dynamic experiments on computers, are more relevant today than the familiar scale models of the city planner. They are an extremely useful tool for perception of present trends as well as prediction of future likelihoods.

The present tax policies and legal structure within the cities, in the view of the author, are largely responsible for the urban decay so visible throughout America. Past efforts to prevent or eradicate this blight have produced little success. Policies for reviving urban areas have failed because complex systems, unlike simple ones, defy solutions suggested by human reason and intuition. Most systems that people ordinarily are required to deal with are simple. For example, in the "traffic systems," the driver quickly learns when and how to apply the brake. Simple systems tend to be linear and to involve only one feedback loop. Complex systems, of which the city is one, are nonlinear and involve multiple feedback loops. In simple systems, cause and effect are closely related in time and space. Whereas in complex systems, they may be so far apart as to obscure their connection. Unresponsive as they are to common-sense approaches, complex systems are examined through simulation on computers that can process the detailed interactions between parts of the system.

Intuitive measures proposed for short-term urban improvement, Professor Forrester points out, often intensify the very problems they are intended to alleviate and thus are detrimental in the long-run. One of the striking conclusions he reaches is that underemployed training programs and low-cost housing programs, although they seem promising, may actually hasten degeneration, while demolishing slum housing and replacing it with industry to create jobs - a policy that is widely frowned upon as inhumane - tend to set in motion a number of desirable long-term trends.

Two types of simulation models are developed in the book. One, a growth model, generates the life cycle of an urban area from its founding through growth to its arrival at a state of stagnation and decay after a period of 250 years. The other begins with the resulting dressed conditions and is used to examine how various policies would alter the condition of the urban area over the next 50 years. The variables include different levels of employment, housing, and industry.