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 The following excerpt was taken from the Transportation Planning Handbook published in 1992 by the Institute of Transportation Engineers (pp. 100-102). Data processing of information describing the urban area and the transportation system requires identifying that information with a numerical code to facilitate automated retrieval. To do this the area being studied is divided into small geographic areas called zones, and the boundaries of each zone are drawn on a base map of convenient scale. A unique numerical code, usually consecutive starting with number one, is assigned to each zone. . . . The time, cost, and capacity for computer processing dictate that there should usually not be more than 1,000 analysis zones. . . . In large metropolitan areas, the recommended limitation on the number of zones may yield zones that are too large for detailed transportation analysis. The approach that has been chosen by some agencies to overcome this difficulty is to define zones that are small enough to perform the most detailed analysis anticipated. These small zones are aggregated to larger zones of an appropriate size for analyses requiring less detail.
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