Four-stage Model Revamped in VISUM
New four-stage model in VISUM: Become a beta tester
More and more of our clients are using VISUM to build full demand models instead of just using it for the assignment stage. Following suggestions from these projects we are revising the four-stage demand model within VISUM to bring it in line with the current state of practice. The WebTAG guidelines in the UK are an important driver, but users in North America are requesting much the same functionality.
The requirements can be summarized as follows:
- VISUM should offer a generalized nested logit operation under Calculate – Procedures which spans destination choice (D), mode choice (M), and time-of-day (aka macro time period) choice (T). Logsums are carried up through all those levels, not just within nested mode choice.
- The three levels D, M (itself optionally nested), and T can appear in any order. If D is not present, the whole nested logit operation is assumed to run below (= after) distribution.
- The whole model can be run in either absolute (synthesis from scratch) or incremental (pivot-point) mode.
- For each trip purpose, the user chooses between running destination choice singly or doubly-constrained. If doubly constrained, multiple demand strata (e.g. for different person groups) with the same trip purpose (e.g. home to work) all compete for the attractions (e.g. jobs), i.e. exhaust them collectively.
- For each trip purpose, the user chooses between running the model in PA form (whole round-trips) or OD form (single trips).
Secondary requirements concern usability:
- Procedure parameter dialogs should be more user-friendly and provide a better overview.
- Linking I/O to VISUM objects (matrices, zone attributes) should be less error-prone.
A prototype of the new functionality is available now, and we are keen to see it used on a few pilot projects, so we can round out the functionality before the official release in 2012. The VISUM product management team is fully committed to assisting intending pilot users. If you would like to participate in the pilot stage please contact Klaus.Noekel@ptv.de.
