General Modelling Approaches: Top-down or Bottom-up?
Most of the models described in this book include detailed transport networks and disaggregated data on travel patterns — mode of travel, journey purpose etc. In some cases the data are also disaggregated in terms of social characteristics — household type, household income etc. Information on the location of the origins and destinations of the trips made by people and goods provides the basic travel demand matrix. These are assigned, by mode, to the networks used for these trips. The approach to building such models starts with data collection, and proceeds through matrix estimation, model calibration, checking, validation and refinement until finally the model is ready for use in testing alternative policies, plans or strategies. Having been extensively validated (and cost a lot of money to develop?) the model commands a high degree of confidence.
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