VISSIM 5.30-05 User Manual

12 Dynamic Assignment
12.3 Building an Abstract Network
12.3.1 Parking Lots and Zones
When using Dynamic Assignment travel demand is not specified by using
vehicle inputs on selected links with a given volume but in the form of an
origin-destination matrix. To define travel demand using an origin-destination
matrix, the area to be simulated is divided in sub-areas called zones and the
matrix contains the number of trips that are made from all zones to all zones
for a given time interval. Thus the dimension of the matrix is: (number of
zones) x (number of zones).
To model the points where the vehicles actually appear or leave the road
network, a network element Parking lot is used. A parking lot belongs to a
certain planning zone, i.e. trips originating from this zone or ending in this
zone can start or end at this parking lot. A zone can have more than one
parking lot. In that case the coming or going traffic can use any of the parking
lots belonging to a certain zone. The total originating traffic of a zone is
distributed to its parking lots according to user defined relative flows. One
parking lot can belong to one zone only.
The distribution of destination traffic to the parking lots is computed by a
choice model explained in section 12.7.2.
Traffic starting at a parking lot is similar to traffic generated by vehicle inputs,
but the composition of the traffic is not explicitly specified for the parking lot.
Instead the vehicle composition is defined with the OD matrix that generates
the vehicles entering at the parking lot. However, the desired speed for the
vehicles leaving the parking lot is not taken from the distribution defined in
the vehicle composition with the matrix, but it is taken from desired speed distributions
defined locally at the parking lot. It is possible to define desired
speed distributions for different vehicle classes at the parking lot, and there is
always a default distribution that covers all vehicles that are not included in
one of these classes. The reason for the local definition of desired speeds at
the parking lot in contrast to define it globally for the matrix is to provide a
way to model the correct speed limit on each road where traffic is originating
from.
Three types of parking lots are offered, which can be used for Dynamic
Assignment. The three types show different driving behavior of the vehicles
entering the parking lot:
► Zone Connector: Entering vehicles do not slow down and are just
removed from the network (parked) as they reach the middle of the
parking lot. Thus the entry capacity is not restricted. This type should be
used to model origin and destination points where traffic enters or exits
the network without using real parking. Typically this is the situation at
the borders of the modeled network.
► Abstract Parking Lot: On approach of a parking lot vehicles slow down
until they come to a stop in the middle of the parking lot. Then the vehicle
596 VISSIM 5.30-05 © PTV AG 2011