Abstract
The modified gravity model is a trip distribution model that differs from the existing gravity model in that two sets of friction curves are used. The selection of the curves from a family of curves is controlled by the requirement that row and column totals of the distribution origin-destination matrix are equal to the assumed trip and values. The paper compares the two models and shows that the existing gravity model is a special case of the more general modified gravity model. The results of one calibration for work trips for a test city are presented.