Using dynamic assignment

When using dynamic assignment, traffic demand is not specified by using vehicle inputs with a given traffic volume for specific links, but in the form of one or more origin-destination matrix/matrices. Thus you can simulate a road network without having to manually create routes and vehicle inputs. In origin-destination matrices, you specify the starting and end points of trips and the number of trips between these locations. Thus the dimension of an origin-destination matrix is the squared number of zones.

In Vissim, the dynamic assignment is done by an iterated application of the traffic flow simulation.

Dynamic assignment glossary

The following terms are used in relation to the dynamic assignment:

• Routes: Static route sequences defined manually as vehicle routes (Modeling vehicle routing decision, vehicle routes and partial vehicle routes)
• Paths: edge sequences computed using dynamic assignment (Attributes for path search), (Attributes for path selection)
• Costs: the exact financial costs. This is the component of the general cost that is neither travel time nor travel distance. The term costs is also used for general costs if the context precludes any chance of confusion (Attributes for calculating costs as a basis for path selection).
• General cost: is the weighted sum of travel time, travel distance and financial cost. The general cost serves as a basis for the utility function of path selection in the decision model (General cost, travel distances and financial cost in the path selection).
• Travel time: the average time the vehicles needs for passing a path or an edge in the current simulation.
• Smoothed travel time: is computed by exponential smoothing of the travel times measured in the course of iterations. The smoothed travel time is the one that is used in the general cost function.
• Expected travel time: is used if we want to express the difference between the travel time that is actually measured in the simulation run and the travel time that is expected in the path selection decision.

During dynamic assignment, you can use microscopic or mesoscopic simulation to simulate the entire network (Using mesoscopic simulation). If you choose to use dynamic assignment with mesoscopic simulation, you can also simulate one or multiple sections of your Vissim network microscopically (Using hybrid simulation).

Dynamic assignment takes the toll pricing calculation into account (Calculating toll using dynamic assignment).

Examples

• You will find a simple use case of dynamic assignment in the 3 Paths.inpx file, in the directory:

..\Examples Training\Dynamic Assignment\3 Paths

• You will find a simple use case of dynamic assignment with a detour in the detour.inp file, in the directory:

..\Examples Training\Dynamic Assignment\Detour

• You will find a simple use case of dynamic assignment for a drop-off zone at an airport terminal or a railway station in the file Drop-off zone.inpx, in the directory:

..\Examples Training\Dynamic Assignment\Drop-off Zone.Trip Chains.inpx

• You will find a use case of dynamic assignment with real parking lots in the file Parking Search - Real Parking Spaces.inpx, in the directory:

..\Examples Training\Dynamic Assignment\Parking Search - Real Parking Spaces.inpx