Influencing path search and path choice
Despite ample exact modeling of the road network, the traffic control and the demand, the result of the dynamic assignment can deviate from the situation which is observed in reality. This can result from the fact that the decision model in Vissim cannot account for all influences, due to their abstractness, which affect the driver in reality. In these cases, the use of parts of road networks are changed during the path selection of the dynamic assignment.
Modeling cost surcharges
If sections of the road network experience more or less traffic than expected, this can be modified via the use of cost surcharges. In order to make a link or connector less attractive for the path choice, set its attribute Surcharge 1 or Surcharge 2 to a larger value (Attributes of links). This value is added to the static cost of each edge including that link or connector. Surcharge 1 is multiplied with the cost coefficient of the vehicle type (Defining path selection behavior with general cost function). Surcharge 2 is simply added to the general cost (coefficient = 1).
If there are connectors from and to a link, the edge includes multiple parts of the link and the surcharge is added multiple times. Therefore, assign surcharges to connectors that lie entirely on an edge, for example, on a turn of a dynamic assignment node.
When using the attributes for Static cost and surcharges, place nodes very close to, but in front of, origin parking lots and very close to, but behind, destination parking lots. This way you avoid that dynamic assignment is distorted by surcharges or costs per kilometer on links before origin parking lots or after destination parking lots.
Blocking edges
You can block edges for the path search of the dynamic assignment and thereby displace traffic from specific network sections (Excluding edges from dynamic assignment). Blocked edges are shown in red.
Limiting the number of paths
In principle, the number of paths which are found in the iterations are not limited. By default, all paths which are found are collected in the path archive. The paths are thereby available for the path selection. A consequence of this can be that from a few attractive paths only a few vehicles will be distributed, even when much better paths are found in later iterations and the use of old paths is very unlikely in reality. These paths use up memory space and calculation time. You can enter one of the two following values and thereby limit the number of applied paths per OD relation (Attributes for the trip chain file, matrices, path file and cost file):
| Element | Description |
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Reject paths with too high total cost Max. extra cost compared to the best path |
Threshold in % for the cost difference as a criterion for the path search |
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Limit number of paths Max. number of paths per parking lot relation |
Maximum number of paths per parking lot relation as a criterion for the path search |
The limiting of the absolute number of paths can be unsuitable in the network if there is an alternative path for some origin areas to destination areas, which also should be used and for other relationships for which few paths are possible. In this case the limiting of the cost difference between the best and worst path can be the suitable solution. Thereby the paths which are discarded are those which, via a preallocation of unused edges at 0.1 seconds, appear particularly attractive. If it is later realized, after a number of vehicles have driven there, that these paths are much more expensive than others, they can be discarded again without any adverse consequence.
By default, the path file is recorded at the beginning of every iteration. Within the respective time interval, for each OD relation, none of the paths are used for which a cost difference in comparison to the cheapest path for all evaluation intervals is larger than the specified value. This means these paths can be used in later time intervals and/or iterations, without having to search for them again as a shortest path.
Closing paths with vehicle routes of the type Closure
You can also confine the path search for dynamic assignment by marking a specific sequence of links as vehicle routes of the type Closure in the network. To do so, in the Network Editor, define a route closure like a static route with a routing decision marker of the type Closure for a destination section (Defining parking routes).
For the dynamic assignment, a path which contains a route closure is only used when it is the only path which is found as a result of the shortest path search for a parking lot OD relation. In the event that no better paths are found, which correspond to a lower generalized cost, vehicles of the dynamic assignment can use a path which contains a route closure.
If in principle a specific sequence of links should not be used, a complete edge or complete turn relation in the node is blocked (Creating and using edges). You can change the node polygon and determine the desired link sequence from a complete turn relation from the node entrance up to the node departure or block all turn relations which contain this sequence of links.
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Notes:
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