Several methods are used for trip distribution including the gravity, growth factor, and intervening opportunities models.
The gravity model is most utilized because it uses the attributes of the transportation system and land-use characteristics
and has been calibrated and applied extensively to the modeling of numerous urban areas. The model assumes that the
number of trips between two zones is directly proportional to the number of trip attractions to the destination zone and
inversely proportional to a function of travel time between the two zones, as demonstrated in Equation 1.