All assignments in Visum in PrT as well as in PuT are path based, meaning that possible paths in the assignment are calculated for each origin-destination relation and loaded with a demand share. All other results, especially the volumes of the different network objects and the skim matrices are derived from these loaded paths. Paths are therefore the central result of the assignment procedure.
In Visum the definitions path (PrT path and PuT path), PuT path leg and PrT paths on link level are used. PuT paths are thus described with a sequence of PuT path legs. Link-based PrT paths display all links which lie on a PrT path.
On the basis of assignment results, using paths you can execute detailed evaluations, such as flow bundles (Flow bundles), or verify the assignment results. As an option, Visum saves the assignment of paths found (User Manual: Settings for saving PrT assignment results).
Editing paths in PrT (PrT path object)
In PrT, the user can manually edit paths. New paths can be inserted and existing paths can be modified. Both the course of PrT paths and their volume can be modified by the user (User Manual: Properties and options of PrT paths). These paths are also available in the usual procedure (such as ICA or flow bundle calculations) like those paths created by a Visum assignment. Beforehand however, they have to be converted into demand segment paths, using the procedure convert paths. Furthermore, multiple so-called path sets can be maintained parallel in a network. Path sets thus combine multiple paths to a group. The following use cases occur, editing paths manually:
- Creating an own assignment result, either by creating a network file in a text editor or interactively by editing paths.
- Editing assignment results calculated by Visum. This may occur interactively by editing the path course in the network editor or by editing the path volume in the path list. On the other hand, the paths can be written as network files and edited in a text editor.
- Maintaining different flow bundle results as path sets. Each path set then contains the result (the paths) of one flow bundle calculation.
- Overwriting a selected section of the assignment result with external data. This is how only paths which start in this planned residential area can be edited manually and the rest of the assignment maintained in a transportation analysis.
- Distributing a matrix on paths. For a given matrix and given paths, the matrix values are distributed to the paths. This enables you to replicate the trip distribution and quickly update the manual assignment.
There are two procedures for handling PrT path objects, which can be integrated into calculation processes (User Manual: Managing PrT paths and sets of paths):
- Converting paths (User Manual: Converting paths). The procedure can be used for example, to replace one assignment result with another. There are the following possibilities:
- Converting assignment result to path set
- Converting path set to assignment result
- Converting path set to path set
- Converting assignment result to assignment result
- Distributing a matrix to paths (User Manual: Distributing a matrix on paths). Based on a matrix and paths, the trips of the matrix are distributed to the paths. This enables you to modify the demand on the level of OD pairs and then distribute the new demand to all existing paths of the OD pair, in proportion to the previous shares. Distribution is carried out with the attribute ShareOfPathTarget. The attribute can be defined for each path by the user. For each OD pair of a path set the attribute ShareOfPathTarget is first added up (total weight) on all paths.
Where P is all paths in a path set of origin O to destination D. If e.g. there are five paths from zone A to zone B, the ShareOfPathTarget of the five zones is added together.
The volume of an individual path p then results from the following equation.
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Note: In the directory C:\Users\Public\Documents\PTV Vision\PTV Visum 2022/Examples, you can find an example of use on this topic. The PrT Paths example shows the use of the network object PrT paths to compare travel times from assignments with observed data. |
