VISSIM 5.30-05 User Manual

12 Dynamic Assignment
12.7 Optional Enhancements of the Model
12.7.1 Multi-class Assignment
Multi-class assignment is the simultaneous
assignment of different interacting road user classes
on the same network. The road user classes in
general have different route choice behavior and
they can access different subsets of the road
network. Examples of user classes are commuters,
business travelers, local drivers, foreign drivers etc.
To model different route choice behavior the parameters α, β and γ of the
general cost function can be defined separately for each vehicle type. Thus it
is possible to model e.g. drivers that are willing to pay tolls to gain time, and
other drivers that do not want to pay and accept longer distances or travel
times.
The parameters can be set in the Cost Coefficients window that can be
reached from the vehicle type window by pressing the button COST
COEFFICIENTS. When defining the coefficients it is important to take into
account the units of cost components to get the right scale of the result:
Travel time is in seconds and distance is in meters, Link Cost has no implicit
unit. So if e.g. link cost is given in Dollar per kilometer and shall have a
significant influence, the coefficient must be large enough to bring the whole
term to the same order of magnitude as the travel times in seconds.
The second aspect of multi class assignment is the selective accessibility of
the road network, that means that not all different vehicle types are allowed
in all parts of the network. This feature could be used e.g. to model local
drivers with full knowledge of the network and foreign drivers who know only
the major roads.
In VISSIM access to parts of the network is controlled by the connectors. A
connector can be closed for selected vehicle classes. The route search will
then build no routes containing that connector while building routes for that
vehicle type. See section 6.3.2 for an explanation how to use connectors.
12.7.2 Parking Lot Selection
The travel demand given in the OD matrix refers to zones for destinations. In
the VISSIM network, zones are represented by one or more parking lots. If a
zone is represented by more than one parking lot, the driver has to choose
one of them before he chooses the route.
Parking lot choice is another example of a discrete choice problem, so we
have to define the set of alternatives, the utility function and the decision
630 VISSIM 5.30-05 © PTV AG 2011