For some projects particularly in connection with demand modeling (Demand model) a (time-consuming) timetable-based assignment is carried out several times within the same procedure sequence.
The PuT skims are determined first, for example, then the mode choice is calculated and finally the actual PuT assignment is performed. Since during the PuT skim calculation all connections are already determined, it is reasonable to use them in the PuT assignment. Thus the assignment includes only the step connection choice (Connection choice).
The timetable-based assignment has such a modular structure that search and choice can be performed independently of each other and the found connections can be stored for later use. This provides the following advantages:
- Replacing a connection search by a pre-calculated path file or by paths taken from an already existing assignment saves lots of time. In many networks, the search is the most time-consuming step of the computation. Possibly the peak memory requirements can be reduced during the assignment.
- Paths from external sources can be taken as inputs to the Visum assignment. This allows for a user-defined heuristic during search and choice without resigning other advantages of the Visum assignment.
- Variants of assignment results can be stored via connection export and later be read in for post-assignment analyses, if required. It is no longer necessary to use several version files.
Image 165: Flow chart of a timetable-based assignment
The Image 165 indicates when paths can be read in from file or output to file in the timetable-based assignment. The following options are provided:
(1) External choice / Connection import
Connections with volumes are imported from file and stored like an assignment result, therefore as paths and network volumes (Use case (1): External choice / Connection import).
(2) Connection export
In order to provide the external choice with data, connections can also be exported. You can do so with or without volumes and optionally choose fare points, fares or user-defined attributes (Use case (2): Connection export).
(3) Using existing connections for the search
Instead of the connection search in Visum, the procedure can also use existing connections as a basis. These connections can be the result of a previous assignment or be stored in a connection file. This feature is of major interest if the search parameters have not been modified, but a choice is to be carried out with different settings (Use case (3): Using existing connections for the search).
It is possible, but not necessary, to deactivate the option Calculate assignment for the connection export. As for a pure skim calculation, neither paths not volumes are stored in this case. Existing assignments of the selected demand segments are retained.
