VISSIM 5.30-05 User Manual

12.8 Assignment Control
12.8.1 Path Evaluation File
Assignment Control
To analyze the results of the Dynamic Assignment, the most important
information is to know the paths found, their cost and how it is computed,
and how much of the traffic volume was assigned to them. This kind of
information is offered for output in the path evaluation file. Cf. section 11.21
for a detailed description of the file format and how to configure the
evaluation.
12.8.2 Iteration Control
The Dynamic Assignment is computed by running the simulation for the
same model repetitively. During the iterations, information about routes in the
network and about travel times on the edges of the road network is collected.
This information is stored in two files, the cost file (BEW) and the path file
(WEG). These files represent the current state of the assignment. The
names of these files are not generated automatically like most of the
evaluation files but can be set by the user in the Dynamic Assignment
window. That way different sets of assignment states can be stored.
Writing of the cost and path files during simulation can be disabled in the
Dynamic Assignment window by deactivating the options Store Costs and
Calculate and Store Paths. This is appropriate for example if the assignment
has converged and the route choice shall not be changed during the
following simulations. To prevent accidental overwriting, VISSIM
automatically deactivates these two options whenever a network file is
opened which is linked to existing cost and path files. If VISSIM is run from
the command line in batch mode with a number of runs specified, VISSIM
will create and/or overwrite these files automatically.
The number of routes available increases during the iterations. In the first
iterations, only few routes are known for each origin-destination-pair. This
may lead to unrealistic congestion on these routes because the volume
cannot be distributed on enough different roads. This congestion will disappear
in later iterations when more routes are found but convergence is
slowed down because of the exponential smoothing of the measured travel
times. To avoid these start-up congestions it is recommended to load the
network with less than the full travel demand during the first iterations (e.g.
10-20%). To continue from there, there are two options:
1. Delete the cost file and run the next iteration with full demand load.
Because of the previous iterations, now most of the paths for every OD
pair are available and thus traffic demand will be distributed over these
paths.
User Manual © PTV AG 2011 637