Attributes for path search

Path search

On the Choice tab, in the Path choice model section, select the Stochastic assignment (Kirchhoff) option or Equilibrium assignment.

• Search new paths: Before path selection, Vissim performs a path search (Path search finds only the best possible path in each interval).
• Search alternative paths (Performing an alternative path search)
• Keep paths for OD relations with zero demand: A parking lot-parking lot relation is without demand, if none of the OD matrices or the trip chain file contain a demand for this relation. OD matrices and trip chain file must be selected (Attributes for the trip chain file, matrices, path file and cost file).

 If this option is selected, all paths for OD pairs with zero demand will be preserved, i.e. no paths will be discarded, when the simulation begins. Vissim will not discard any paths if the volume in one of the selected OD matrices or the trip chain file is rounded down to zero in the current simulation run because a very low total volume < 1 was defined.

If, in addition, you select Search new paths, the software will search for new paths not only for parking-to-parking OD pairs with demand but also for OD relations without demand between parking lots.

 If Keep paths for OD relations with zero demand is not selected, all paths for OD pairs without demand will be discarded when the simulation starts.

The option Keep paths for OD relations with zero demand is ignored, if on the Choice tab, in the Path choice model section, Use volume (old) is selected. No paths will be searched for. All paths will remain unchanged upon simulation start.

If in the Path selection type section, Decide repeatedly is selected, the software will not search for new paths for OD relations with zero demand.

Path selection type

If there are several paths a vehicle can use to get from the origin parking lot to the destination parking lot, the vehicle can decide on the path it wants to take at the following times:

• Decide at start only: The vehicle chooses its path to the destination parking lot at the origin parking lot or at a dynamic vehicle routing decision.
• Decide repeatedly: On the Choice tab, in the Path choice model section, the option Stochastic assignment (Kirchhoff) or Equilibrium assignment must be selected. The vehicle chooses its path dynamically while on the road. Before a vehicle reaches a node, Vissim determines the best route to the vehicle's destination parking lot for each turn relation of the node. Based on the Kirchhoff model for generalized costs, the vehicle randomly chooses one of these paths. These costs are aggregated if the path follows a toll route and is therefore a combined path. This procedure is repeated at every node, until the vehicle has reached its destination parking lot. When it comes to lane change, the vehicle accounts for the driving behavior attribute Look ahead distance and at least the next three nodes. As soon as the vehicle has reached its destination parking lot, the path used is saved to the path collection. If the vehicle takes a toll route, it continues to drive on the toll route until it ends.

Select this option, and in the Cost tab, click Previous time interval, current simulation run to perform dynamic assignment with a single simulation run, without any additional iterations. In this simulation run, all possible paths (without loops) can theoretically be used.