PTV Visum - Frequently Asked Questions (FAQs)
Why are connectors by shares not met after an assignment (PrT)?
The Connector's shares can be disabled with the setting:
Calculate -> General procedure settings -> PrT settings -> Connector weights apply to -> Total trips (MPA off).
When using 'Each single OD pair' instead, the connector's shares are considered.
You can distribute the demand among a zone's PrT connectors A) absolutely (freely) or B) proportionally (by shares):
A) If a zone has multiple connectors and the zone is set to 'Absolute', this will use the connectors according to shortest paths. It can be that a connector does not get any volume at all.
B) To meet connector shares, use the zone setting 'By shares'. In this case choose between 'Total trips (MPA off)' or 'Each single OD pair':
1) 'Total trips (MPA off)' is a volume-dependent consideration, the connectors' capacities are taken according to the shares and the impedance of connectors is calculated by a VDF. This way, you usually won't reach exact shares, which does however depend on how 'hard' the VDF is set.
2) 'Each single OD pair' is in fact a multi-point assignment (MPA), the demand of zones with proportional demand distribution is divided appropriate to these shares (thus the demand matrix is expanded and disaggregated) before the assignment. After the assignment, the matrix is aggregated again. This way, the connectors will meet exactly their shares. But caution, this takes extra calculation time and memory capacity.