Equilibrium efficiency and optimum are fundamental concepts that characterize the form of ideal cities. Such forms, coupled with the closed' - open' distinction, provide a rich gamut of urban structures. Yet transcending this diversity of types and individualistic attributes there is a more general, unified level of analysis. Long (1971) has worked at this level, and the trend is continued in this paper. Externalities, policies, and public goods are fundamental issues underlining the recent strong concern for normative analysis. Such issues are typically related to questions of environmental quality. Indeed the impact of environmental quality upon urban structure is decisive: the message emerges clearly as the main conclusion of this essay. The gist of the analysis concerns relations among environmental quality, composition of society, and spatial choice. These relations are general in the sense that they apply both to positive and to normative models of cities.
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