VISSIM 5.30-05 User Manual

12 Dynamic Assignment
12.1 Introduction
In the preceding chapters, the simulated vehicles followed routes through the
network that were manually defined by the user, i.e. the drivers in the
simulation had no choice which way to go from their origin to their
destination. For a lot of applications that is a feasible way of modeling road
traffic.
However, if the road network to be simulated becomes larger it will normally
provide several options to go from one point in the network to another and
the vehicles must be distributed among these alternative routes. This
problem of computing the distribution of the traffic in the road network for a
given demand of trips from origins to destinations is called traffic
assignment and is one of the basic steps in the transport planning process.
Traffic assignment is essentially a model of the route choice of the drivers
or public transport users in general. For such a model it is necessary first to
find a set of possible routes to choose from, then to assess the alternatives
in some way and finally to describe how drivers decide based on that
assessment. The modeling of this decision is a special case of what is called
discrete choice modeling, and a lot of theory behind traffic assignment
models originates from the discrete choice theory.
The standard procedure in transportation planning is the so called Static
Assignment. Static here means that the travel demand (how many vehicles
want to make trips in the network) as well as the road network itself is
constant in time. However, in reality travel demand changes significantly
during the day, and even the road network may have time dependent
characteristics, e.g. signal control may vary during the day. To consider
these time dependencies, Dynamic Assignment procedures are required.
The motivation to include route choice in a simulation model like VISSIM is
twofold:
► With growing network size it becomes more and more impossible to
supply the routes from all origins to all destinations manually, even if no
alternatives are considered.
► On the other hand the simulation of the actual route choice behavior is of
interest because the impact of control measures or changes in the road
network on route choice are to be assessed.
The small example DETOUR.INP which can be used as an introduction to
Dynamic Assignment is enclosed with the VISSIM installation and located in
..\EXAMPLES\TRAINING\DYNAMICASSIGNMENT\DETOUR\.
592 VISSIM 5.30-05 © PTV AG 2011