Simulation-based dynamic assignment (SBA) is a dynamic assignment procedure that accounts for node impedances and allows users to model the forming and dissolving of queues over time. The supply and demand may be varied over time.
Unlike other dynamic assignment procedures in Visum, the network is loaded with demand based on a simulation. This means that individual vehicles are simulated and a simple car following model is applied to have the vehicles follow the paths they are assigned. The algorithms used are based on the work of M. Mahut (Mahut, M.: A discrete flow model for dynamic network loading, PhD thesis, University of Montreal, Canada, 2001).
The assignment is an iterative procedure that includes the steps route search, network balancing and the simulation. These steps are carried out until a relative gap or the maximum number of iterations is reached.
This procedure can be based on existing assignment results, provided that the balancing time intervals and the demand segments do not change. Generally, a new equilibrium for a changed demand and/or modified network can be achieved faster by using an existing start solution as a basis. This option allows you to model and analyze use cases such as the impact of planned and unplanned events. When the conditions of supply change, unplanned events can be modeled as a one-time passing of already found paths, e.g. based on existing results, i.e. without having to conduct a new path search. In such a case, during the first iteration, it is merely checked whether existing paths are still permitted and the volumes of existing paths are scaled to the demand of the respective balancing time interval. For planned events, it may be assumed that a state of equilibrium will be reached again, i.e. based on the original condition, iterations can be performed until a state of equilibrium is reached under the new conditions.
The procedure is suited for medium-sized and large networks whose network model and demand are suitably modelled for dynamic applications. If junction control data and node geometry have not been defined according to real conditions for all parts of the network, suitable attribute values of parameters must be defined to ensure that route search and choice deliver consistent results.
The vehicles from the simulation-based dynamic assignment can be visualized in a special view window (Visualization of vehicles from the simulation-based dynamic assignment).
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Note: In the directory C:\Users\Public\Documents\PTV Vision\PTV Visum 2021/Examples, you can find examples of use on this topic. The PrT Assignment SBA example illustrates the use of the SBA method including the description of the steps to set up a model for SBA. The SBA MultiRun example describes an application in scenario management for multiple execution of SBA with different random numbers. In the same folder, you will also find the document SBA_BestPraticeGuide. |
