In the case of the trip distribution process, the additional information that is added is some form of impedance. For
example, the original gravity model considered that the likelihood of trips between two zones was proportional to the
inverse of the square of the distance between the two zones. Since that time, many more sophisticated forms of
impedance have been considered, but for the purposes of this discussion, all of these variations can be generalized as
being of the form Fij, where Fij = f(cij) or the generalized cost of inter-zonal travel. What is less obvious, however, is the
fact that the use of this set of impedance factors Fij, is essentially equivalent to the use of a seed matrix tij.