VISSIM 5.30-05 User Manual

12.2 Principle
Principle
The Dynamic Assignment procedure in VISSIM is based on the idea of
iterated simulation. That means a modeled network is simulated not only
once but repetitively and the drivers choose their routes through the network
based on the travel cost they have experienced during the preceding
simulations. To model that kind of “learning process”, several tasks are to be
addressed:
► Routes from origins to destinations must be found. VISSIM assumes that
not everybody uses the best route but that less attractive routes are used
as well, although by a minor part of the drivers. That means not only the
best routes must be known for each origin-destination relation but a set
of routes. Ideally we would have the set of the k best routes but there are
no efficient methods to compute this set of routes directly - at least not in
a way that makes sense for traffic assignment. The solution adopted in
VISSIM is to compute best paths in each repetition of the simulation and
thus to find more than one route because traffic conditions change during
the iteration. During the iterated simulations VISSIM builds a growing
archive of routes from which the drivers choose. See section 12.6 for a
detailed description of how routes are computed.
► The routes must have some kind of assessment on which the drivers
base their choice. In VISSIM for all routes the so called generalized costs
are computed, i.e. a combination of distance, travel time and “other”
costs (e.g. tolls). Distance and costs are defined directly in the network
model but travel time is a result of the simulation. Therefore VISSIM
measures travel times on all edges in the network during one simulation
so that the route choice decision model in the next simulation can use
these values.
► The choice of one route out of a set of possible routes is a special case
of the more general problem called “discrete choice modeling”. Given a
set of routes and their generalized costs, the percentages of the drivers
that choose each route is computed. By far the most frequently used
mathematical function to model that kind of choice among alternatives is
the Logit model. VISSIM uses a variant of the Logit model to handle
route choice. See section 12.6.2 for a detailed description.
The VISSIM road network model is very detailed in order to allow an exact
reproduction of the traffic flow in a high resolution in time and space. All of
the three tasks above do not require such a detailed model of the network,
e.g. the choice which route to take from one side of the city to the other does
not consider how the intersections actually look like or on which lane the
vehicles travel.
Assignment related problems always refer to a more abstract idea of the
road network where the intersections are the nodes and the roads between
the intersections are the edges of an abstract graph. The assignment
procedures can operate much more efficiently on this type of graph and this
level of abstraction is more appropriate even for the human understanding of
the problem.
User Manual © PTV AG 2011 593