The standard 4-step model generally includes the steps trip generation, trip distribution, mode choice, and assignment. In Visum, two variants are offered that allow you to calculate a 4-step model:
- Fixed, sequential performance of the steps trip distribution and mode choice in separate procedures (4-step model with sequential calculation)
- Flexible sequence of options, e.g. destination and mode choice in model structures (Nested demand model)
The two variants have in common that both trip generation and assignment are carried out in separate steps, either before or after calculation of the demand matrices. To calculate the matrices, you must define utilities. To do so, you need skims from assignments as well as other coefficients and constants. The steps trip distribution, mode choice or calculation within the scope of nested demand and assignment be repeated until a specified termination criterion is met. The condition for termination can be determined through the change of demand matrices, of impedances in the network or through a gap.
Using the procedure “Go to the procedure”, you can check whether the termination criterion is met and control the repeated sequence of the loop (Go to procedure).
For both variants, a demand model of the type standard 4-step must be defined in which the available modes and demand strata (if necessary obtained through combinations of person groups and activity pairs) are specified.
Both variants differ in terms of the calculation of demand matrices.
Topics
- 4-step model with sequential calculation (overview of procedure)
A description of trip generation and the individual steps within sequential calculation can be found in:
For a description of the Nested demand model, see:
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Note: In the directory C:\Users\Public\Documents\PTV Vision\PTV Visum 2021/Examples, you can find an example of use on this topic. The Demand 4-step example describes the structure and use of the standard 4-step model for demand calculation. |
