Editing Trip distribution (section)

=== Congestion ===

One of the key drawbacks to the application of many early models was the inability to take account of congested travel time on the road network in determining the probability of making a trip between two locations. Although Wohl noted as early as 1963 research into the feedback mechanism or the “interdependencies among assigned or distributed volume, travel time (or travel ‘resistance’) and route or system capacity”, this work has yet to be widely adopted with rigorous tests of convergence, or with a so-called “equilibrium” or “combined” solution (Boyce et al. 1994). Haney (1972) suggests internal assumptions about travel time used to develop demand should be consistent with the output travel times of the route assignment of that demand. While small methodological inconsistencies are necessarily a problem for estimating base year conditions, forecasting becomes even more tenuous without an understanding of the feedback between supply and demand. Initially heuristic methods were developed by Irwin and Von Cube <ref> Florian M., Nguyen S., and Ferland J. 1975 On the Combined Distribution-Assignment of Traffic", Transportation Science, Vol. 9, pp.&nbsp;43&ndash;53, 1975</ref> and others, and later formal mathematical programming techniques were established by Suzanne Evans.<ref>* Evans, Suzanne P. 1976 . Derivation and Analysis of Some Models for Combining Trip Distribution and Assignment. Transportation Research, Vol. 10, PP 37&ndash;57 1976</ref>