VISSIM 5.30-05 User Manual

● Travel Time on Paths computes
the change of the travel time on
every path compared to its travel
time in the previous run. If this
change for all paths is lower than the
user-defined factor, convergence for
this criterion is detected.
Assignment Control
● For Travel Time on Edges the old and new travel times on edges are
compared in the same way as described for paths above.
● For the option Volume on Edges the absolute difference of old and
new volume on every edge is determined and compared with the userdefined
number of vehicles.
It is recommended to use only one out of the three possible convergence
criteria, preferably the Travel Time on Paths option. Make sure that you set
the convergence criteria not too tight (e.g. by using all three criteria at the
same time). Otherwise convergence is never reached. Especially the
Volume on Edges option might be too tight as it looks for absolute values
which on heavily loaded links differ more than on low-flow links even
though the difference in percentage is the same.
This test for convergence is done at the end of each evaluation interval. If all
selected convergence conditions are fulfilled for all evaluation intervals a
message box shows up at the end of the simulation run. In the case of a
batch run a different message box shows up offering the option to terminate
the batch run.
Another way to observe and control convergence during the iteration process
is offered by the convergence evaluation output file. VISSIM writes to this file
a statistic evaluation of the differences in travel times and volumes for all
edges and paths from the preceding to the current iteration. See section
11.22 for a description of the file format.
The non converging paths in the last iteration can be displayed within the
Paths window (EDIT - AUTO ROUTING SELECTION...). A path does not converge
in a time interval if the travel time percentage difference between the
previous and the last iteration is bigger than the percentage defined in the
convergence window. The Paths window displays the previous and the last
travel time for each time interval where a path is not converging.
12.8.4 Route Search Control and Local Calibration
The Dynamic Assignment process can be controlled by the user in several
ways. Control becomes necessary if the assignment result differs from real
world observation although the road network and infrastructure are modeled
correctly. That may happen because for obvious reasons the decision model
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