in transportation calculations the geographical region of interest is commonly sub-divided into zones but it is desirable that the models of the various parts of the transportation process should not be restricted by any such zone system. the consequences of requiring this independence and the results of not so doing are here examined for the gravity model of trip distribution. a formulation of inter-and intra-zonal deterrent functions is suggested which gives consistent results from gravity model calculations performed over any set of combinations or sub-divisions of the original zones. the lack of such consistency inherent in growth factor methods of estimating zone to zone trips is discussed. some approximate formulae for intra-zonal deterrent factors are deduced and examined. (a)

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