VISSIM 5.30-05 User Manual

Closed edges are shown in red color rather than yellow.
Assignment Control
Restricting the Number of Routes
The number of routes found during the iterations in principle is not restricted.
Therefore, as long as no special action is done, all routes that have been
found are stored in the route archive and are used for traffic distribution. As a
result, to some rather expensive routes some vehicles will still be assigned
even if in later iterations much better routes are found. To avoid this effect,
the number of routes that is used for each OD pair can be restricted. There
are two ways of restricting the number of routes:
► Defining an upper limit for the number of routes
► Defining a maximum of cost difference between the best and the worst
route
Defining an upper limit may not be appropriate in networks where for some
OD pairs many alternative routes exist (and shall be used) and for other
relations only few routes exist. Then the maximum of cost difference
between the cheapest and the most expensive route may be defined. This
method is intended to discard unwanted expensive routes that once have
been the cheapest because some or all edges had no real cost evaluations
yet (i.e. they were initialized with cost value 0.1). The low initialization values
encourage the route choice model to assign traffic to routes where no
vehicles have been traveled yet. Those routes which are found to be much
more expensive (compared to existing routes) after some vehicles have
traveled on them for the first time can be discarded from the route archive
using this method.
The following action is performed every time the data from a path file is
loaded (generally speaking at every start of a new iteration):
For each OD pair separately, out of the set of all available routes VISSIM
finds the cheapest and the most expensive route. Then every route is
discarded for which the excess cost (compared with the cheapest route)
divided by the cost of the cheapest route is higher than the defined threshold
factor for all evaluation intervals.
in the Path Search window
► the threshold factor and
► the upper limit
can be defined, if the particular option is active. It is reached by pressing the
button EXTENDED for option Search new paths in the Dynamic Assignment
window.
Furthermore, the following options are provided:
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