Attributes for achieving convergence

The iteration of the simulation for the calculation of the dynamic assignment can be ended once a stable state has been reached. This is the case when from one iteration to the next the travel time and corresponding congestion on the edge no longer changes significantly. This stability must be reached for all evaluation intervals. The situation does not change within one evaluation interval from iteration to iteration. The situation can, however, change within an iteration from one interval to the next because the demand and the network control is time dependent.

1. On the Traffic menu, click > Dynamic Assignment > Parameters.

The Dynamic Assignment: Parameters window opens.

2. Select the Convergence tab.

3. Make the desired changes:

Element	Description
Travel time on paths	If the option is selected, the change of travel time for all paths is considered in comparison to the previous simulation iteration. This convergence criterion is fulfilled if the percentage change for all paths in all evaluation intervals is smaller than the specified threshold. Default value of 15 %. Required share of converged paths: weakens the convergence condition. Share of the paths in % which must at least have converged for the convergence criterion to be fulfilled. Default value 95 %, default value when loading network files which do not contain this attribute yet: 100 %. To exclude the impact of particularly short paths, slightly reduce the value of this attribute to approx. 90 to 95 percent.
Travel time of edges	If the option is selected, the change of the travel time for all edges is considered in comparison to the previous simulation iteration. This convergence criterion is fulfilled if the percentage change for all edges in all evaluation intervals is smaller than the specified threshold. Default value of 15 %. Locked edges are not taken into account when calculating convergence. Not for edges shorter than: Length in meters is a criterion for edges, which is not checked for convergence Default 20 m. Required share of converged edges: weakens the convergence condition. Share of the edges in % which must at least have converged for the convergence criterion to be fulfilled. Default value 95 %, default value when loading network files which do not contain this attribute yet: 100 %.
Volume on edges	If the option is selected, the convergence criterion is fulfilled if, on all edges, the absolute number of vehicles (traffic volume) in all evaluation intervals in comparison to the previous iteration has not changed more than the specified number of vehicles. Default 15 vehicles. Required share of converged paths/edges: weakens the convergence condition. Share of the paths/edges in % which must at least have converged for the convergence criterion to be fulfilled. Default value 95 %, default value when loading network files which do not contain this attribute yet: 100%. To exclude the impact of particularly short edges, slightly reduce the value of this attribute to approx. 90 to 95 percent.
Required number of consecutive converged simulation runs	Number of converged simulation runs after which Vissim is to end the simulation runs even if a higher Number of runs has been specified in the Simulation parameters.
Behavior upon convergence:	Convergence may already be reached before the number of simulation runs is completed that has been defined in the simulation parameters, in the Number of runs box. In this case, you can choose how Vissim should behave. Exit: Do not perform another simulation run Ask (default): Continue to perform simulation runs. If after another simulation run, convergence is reached, a window opens, allowing you to select one of three options. Complete all runs: Perform the defined number of simulation runs. When convergence is reached, do not open a window with behavior options.

4. Confirm with OK.

Note: In order for the convergence to be reached, note the following points:

Select only one of the three convergence criterion.
Preferably, choose Travel time on paths.
If you select multiple convergence criteria, there is a chance that convergence will never be reached because due to the increasing requirements the convergence criteria could become too strict.
In particular, if you select the convergence criterion Volume on edges, there is a chance that convergence will never be reached because the absolute number of vehicles on the highest volume links fluctuate more than on links with less volume although the percentage deviation is the same.

The test for convergence is carried out at the end of each evaluation interval. When convergence occurs with several simulation runs in a row, the following message opens: All selected convergence conditions are true. Cancel further simulation runs? You can select Cancel if you do not want to perform any further simulation runs or a specified number of simulation runs. Before the start of the simulation runs, you can specify the number of converged simulation runs after which Vissim shall not run simulations any more using the attribute Required number of consecutive converged simulation runs.

Checking the convergence in the evaluation file

You can also check the behavior of the convergence during the iteration in the convergence evaluation file. The convergence evaluation file contains a statistical evaluation of the changes from travel time and volume on the edges/paths from one iteration to the next evaluation (Saving data about the convergence of the dynamic assignment to a file).

Showing converged paths and paths that are not converged

In the Paths list, you can show if a path converged or not using the attribute Converged (Conv). The convergence criterion is fulfilled if the percentage change for all paths in all evaluation intervals is smaller than the specified threshold for the selected convergence criterion (Showing data about paths of dynamic assignment in lists).

Superordinate topic:

Controlling dynamic assignment

Information on editing:

Controlling iterations of the simulation

Setting volume for paths manually

Influencing path search

Evaluating costs and assigned traffic of paths