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 and others, and later formal mathematical programming techniques were established by Suzanne Evans. Stability of travel times A key point in analyzing feedback is the finding in earlier research that commuting times have remained stable over the past thirty years in the Washington Metropolitan Region, despite significant changes in household income, land use pattern, family structure, and labor force participation. Similar results have been found in the Twin Cities The stability of travel times and distribution curves over the past three decades gives a good basis for the application of aggregate trip distribution models for relatively long term forecasting. This is not to suggest that there exists a constant travel time budget. See also Environmental impact of aviation Hypermobility (travel) Footnotes References Allen, B. 1984 Trip Distribution Using Composite Impedance Transportation Research Record 944 pp. 118–127 Ben-Akiva M. and Lerman S. 1985 Discrete Choice Analysis, MIT Press, Cambridge MA Boyce, D., Lupa, M. and Zhang, Y.F. 1994 Introducing “Feedback” into the Four-Step Travel Forecasting Procedure vs. the Equilibrium Solution of a Combined Model presented at 73rd Annual Meeting of Transportation Research Board Haney, D. 1972 Consistency in Transportation Demand and Evaluation Models, Highway Research Record 392, pp. 13–25 1972 Hansen, W. G. 1959. How accessibility shapes land use. Journal of the American Institute of Planners, 25(2), 73–76. Heanue, Kevin E. and Pyers, Clyde E. 1966. A Comparative Evaluation of Trip Distribution Procedures, Levinson, D. and Kumar A. 1995. A Multi-modal Trip Distribution Model. Transportation Research Record #1466: 124–131. Portland MPO Report to Federal Transit Administration on Transit Modeling Reilly, W.J. (1929) “Methods for the Study of Retail Relationships” University of Texas Bulletin No 2944, Nov. 1929. Reilly, W.J., 1931. The Law of Retail Gravitation, New York. Ruiter, E. 1967 Improvements in Understanding, Calibrating, and Applying the Opportunity Model Highway Research Record No. 165 pp. 1–21 Stewart, J.Q.