PTV Visum - Frequently Asked Questions (FAQs)
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How can flow bundle volumes exceed link volumes?
If Blocking back is active, assignment results on links (connectors, (main-) turns and (main-) nodes) are changed, in order to show how much traffic flows behind a bottle neck.
The saved paths are not changed, but this is what flow bundles are using.
If you want to compare flow bundle volumes with link volumes, there are three methods:
A) If you take the attribute 'Volume PrT [veh] (AP)', this will show the traffic volume that passed the bottle neck until the end of the assignment interval only. If you take the attribute 'Volume demand PrT [veh] (AP)' instead, this will show the complete traffic volume.
B) Calculate an approximation of actual flow bundle volumes:
1) Define an UDA ratio for links (turns, main turns) with a formula using attributes of volumes e.g. [Volume PrT [PCU] (AP)] / [Volume demand PrT with base [PCU]].
2) Define an UDA actualVolFlowBundle for links (turns, main turns) with the formula [Volume flow bundle (PRT)] * [RATIO].
3) Modify the graphic parameters so that the UDA actualVolFlowBundle is shown.
- Remark: If a flow was not affected by the bottleneck, it still adds to the attribute [Volume PrT [PCU] (AP)], resulting in overestimating the actual flow.
C) Turn off the blocking back model. Proceed as follows:
1) Calculate -> General procedure settings -> PrT settings -> Blocking back model -> Blocking back calculation active - deactivate.
2) Graphics -> Flow bundle -> Execute.