The blocking back model is no independent assignment procedure, but it modifies the results of previously run assignments or of the running assignment. As an option, the blocking back model can be carried out together with an assignment (Using Visum: Blocking back model settings and calculation).
The blocking back model only applies to PrT transport systems. Assuming that PuT transport systems are either not affected by any congestion or that the timetables used already regard the wait times. PuT-caused congestion on commonly used links can be neglected or can be considered by a base volume.
Since volumes of different PrT demand segments are summed up before entering the queue length, all volumes are invariably converted into car units.
You can calculate the blocking back model post-processed following an assignment. It therefore does not influence the route choice.
- To account for blocking back effects on the route choice during assignment, use the procedure "Assignment with ICA" (Assignment with ICA).
The blocking back calculation can be combined with the following assignment procedures: Incremental, Equilibrium, Equilibrium_Lohse, TRIBUT and any stochastic procedure. However, it cannot be combined with a dynamic assignment procedure.
If an assignment has already been calculated for at least one demand segment, which is not to be recalculated, the blocking back model is calculated prior to the execution of the assignment of the already assigned demand segments. This is to ensure that the values for tcur and tw are consistent with the current network status and to avoid that assignments with a blocking back model share and those without are combined.
Limiting capacity
According to rule 1, the traffic flow from link to link along a route is limited by the capacity of the link and the capacity of the link's ToNode and the capacity of the turn during the blocking back model calculation. In the blocking back model parameters you can select individually, whether link and turn and node capacities are to be regarded. The settings have the following meaning:
- Link capacity restricts the outflow per link. As threshold, either the link attribute Capacity PrT can be used or the summed up Capacity PrT of the outgoing turns. The latter option is only provided for compliance with out-dated versions. It is no longer recommended. It is recommended to use the option Turn capacity instead.
- Node capacity restricts the flow per node (sum of all turn volumes) to the node attribute Capacity PrT. These node capacities are only regarded for traffic flows on secondary links (TModelSpecial = true) towards the node. Traffic flows on major legs therefore also have an effect on crossing routes via secondary links.
- The Turn capacity limits the flow on each turn to the Capacity PrT or Final capacity for assignment with ICA attribute of the turn.
These three options can be combined at will.
If you are using the blocking back calculation to post-process a PrT assignment, you can decide by node, whether the global blocking back model parameter settings are to be considered; alternatively, a node-specific setting (node attribute Use preset blocking back capacity settings = activated) can be used. The node-specific setting is regarded for all turns via this node and for all links leading into this node. To regard the turn capacity only at particular nodes, for example, then exclude the consideration of turn capacities in the global blocking back model parameters and activate the attributes Use preset blocking back capacity settings and Use turn capacity in blocking back at the particular nodes.
