External Zones in the Gravity Model—Some Further Developments

In urban transport studies, the boundaries of the outermost zones are usually set to coincide with the expected periphery of development at the end of the study period. In practice, there often exist considerable traffic flows to and from these zones and settlements outside the urban area. In this note, suggestions are made to include such flows within a consistent entropy maximizing framework. The model accounts for the total worker supply and total job demand characteristics of the external zones, whilst ensuring that the relevant constraints are not violated.