By Tony Hiss

Knopf. 384 pp. $19.95

THE SIMPLE but striking premise of this stimulating book is that the United States, once a country of endless frontiers and wide open spaces, has reached a watershed. The current population of about 250 million already represents five-sixths of the predicted total population 50 years from now -- after that the numbers are expected to stabilize, even to slightly decline. The new world, if not yet old, is, at least, grown up. In the author's words, "Over the next hundred years or so, America will essentially complete itself."

This process of completion will be conditioned by the choices that are made today and, although the author does not emphasize this enough, choices that were made in the past. The future environment will be the result not only (and most obviously) of current decisions about resources and ecology, but also decisions about the shape and character of our cities, and about the way that cities are allowed, or directed, to change. This is not a technical book about zoning, city planning or urban economics, however. What concerns Tony Hiss, a staff writer at the New Yorker, are values: If we are to make the correct choices, he argues, we must decide what sort of environment we want -- and need.

America is -- and will continue to be -- an urban society, and The Experience of Place is, quite properly about cities, and about the city-dweller's experience of the natural landscape. These two subjects form the author's two, interacting themes. The first part of the book offers several examples from New York City such as Grand Central Terminal and Times Square, as well as Brooklyn's Prospect Park, to illustrate how we experience urban places. Some of the most engaging passages in the book take the reader through these famous spots in such a perceptive way that even someone familiar with them will experience new insights.

Cities are a combination of the man-made and the natural or, in the case of parks, of the man-made natural. It is interesting to learn that our fondness for parks may not only be cultural, but also biological. Hiss interviews John H. Falk, who has studied people's landscape preferences in different cultures and has discovered a preference for grassy landscapes, "even among people who had never been in a grassland setting in their lives." Falk speculates that since human beings first evolved in the savannas of East Africa, we may have a generically transmitted predisposition for this type of terrain.

Hiss's purpose here is to understand what it is that makes successful urban places such as Times Square so appealing. To that end he reports on the work of various environmental researchers, and these digressions are an informative reward of this book. The reader learns, for example, of the Berkeley Simulation Laboratory, which has developed a cinematic technique that makes it possible to simulate the experience of walking down a city street, and which can be used to study the effects of zoning changes or new construction on such things as sunlight, views, orientation and general liveability. This technique has already been used by the city of San Francisco in developing its latest set of downtown planning regulations.

MODERN city planning has a checkered history -- remember the urban renewal debacles of the 1960s? According to Hiss, there is a new urban science developing which aims, first of all, to better understand how people use city-places, and to employ this information to "repair" dysfunctional neighborhoods. Employing time-lapse photography, researchers in Frankfurt, for example, have been able to suggest physical changes that significantly improved the downtown commercial district. The changes are based on two observations: first, that people's sense of security in public spaces is connected to identifying small "safe" places where one can sit or stand without getting in another's way; second, that people need reasons for going to a place, and the more reasons they have, the more secure they will feel, and the longer they will spend there. These are prosaic, commonsensical discoveries, but often forgotten by urban designers.

The author's other theme, which comprises the second half of this book, concerns the relationship of the city to the surrounding countryside. If we are all to live in cities, maintaining our ties to the natural environment by en masse excursions on weekends and holidays, we may finally destroy the very thing we desire. Hiss suggests several solutions. One involves preserving the undramatic but valuable working countryside that still exists around -- and sometimes within -- our cities. He gives several examples from various parts of the country -- Rhode Island, Pennsylvania, Oregon -- where regional planning has successfully balanced the needs of economic development and ecological preservation. Another lesson seems to be that whereas conservation in the past focused on large, prominent areas of natural amenity such as wilderness areas, the future will require a finer-grained sensibility that is attentive to more modest, but equally valuable, local resources -- "sweet spots," in the author's memorable phrase. Here is a case where saving the whole will require saving many, many small parts.

Witold Rybczynski is the author of "The Most Beautiful House in the World."