This implies that solving the trip distribution problem, using zonal trip productions and attractions as constraints, together
with a trip impedance matrix, is essentially the same as solving the synthetic O-D generation problem using zone
connector in and out flows as constraints, and utilizing a seed matrix based on Equation 2, which is sometimes utilized in
the gravity model trip distribution procedure. This similarity is very encouraging to most traffic engineers and planners for
the following reasons. First, many planners view the trip distribution process as a well established approach for estimating
O-D matrices, which has a long history and is hard to bring into question. In contrast, synthetic O-D generation is much
less well known, and while many are not familiar with the details of the technique, most have heard of the difficulties
arising from the under-specification problem. The above similarity indicates that planners should not question the validity
of O-D matrices estimated synthetically any more than they question the validity of O-D matrices estimated from the trip
distribution process. Second, if planners are presently utilizing O-D matrices derived from a trip distribution exercise, they
can now recognize that providing additional information about en-route links within a synthetic O-D generation process
can only enhance their matrix accuracy, especially if they use the trip distribution matrix as a seed.