the variation with position of the number and length of trips generated in a uniform circular town is found, on the assumption that the generalised costs of travel per day for each person are constant and that the deterrent effect of separation varies inversely as the separation. it is shown that the average trip length is 35 per cent less than that for a random coupling of origins and destinations in a uniform circular town. the bias that might result from using an approximate method to calibrate a gravity model is also investigated, by applying the approximate method to the theoretical situation described in this note. (a).

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