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aggregate travel model

  A device for estimating the total coverage of distance involved in serving the market from alternative locations. In its more general form this model can be applied to any situation where it is necessary to aggregate all trips made by individual participants in some activity, but the most common context is industrial location.

The aggregate travel model is related to the market potential model (see also centrography; population potential), in that they both compare the advantage of alternative locations with respect to the market, but under different assumptions. The aggregate travel model assumes a market of varying size in different places, but one that is not sensitive to delivered price or distance from the production location. The model seeks the point of minimum coverage of distance, given by:

{img src=show_image.php?name=bkhumgeofm1.gif }

where Ai is aggregate distance travelled to serve the market from plant location i to all n points or areas comprising the market; Qj is sales expected at market j; and Tij is the distance or transport cost between i and j. The market size Q may be assumed to be proportional to some alternative measure, such as per capita income or retail sales, as in the market potential model. Similarly, Tij may be linear distance, actual cost, or distance raised to some power to reflect the actual cost-distance relationship in the prevailing freight rates (see distance decay; transport costs).

The aggregate travel model usually provides figures for the relative advantage of alternative locations. Only if Q actually represents volume of sales and T the real transport cost will the calculation of A give total transport costs for serving the market. As with the market potential concept, figures for aggregate travel can be mapped in the form of a surface. Comparison with a market potential surface derived from the same data reveals differences between the spatial patterns of advantage with respect to serving the market, arising from alternative assumptions as to the nature of the demand situation. (DMS)

Suggested Reading Smith, D.M. 1981: Industrial location: an economic geographical analysis, 2nd edn. New York: John Wiley, 272-4.



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