From a mathematical point of view, this type of simulation basically represents a numerical integration of a system of coupled differential equations. Smaller time steps allow for solutions approaching the exact solution. A larger number of time steps, however, requires more computation. Vissim calculates pedestrians with a time step of 0.05 seconds, i.e. 20 times per second. This value only slightly diminishes the accuracy compared to the exact solution. By comparison, in many projects the impact of uncertain external factors is probably greater, e.g. the real number of pedestrians and their speed.
Vissim's program module for pedestrian movement internally communicates with other modules. These modules for instance generate pedestrians or remove them during simulation, calculate route choice or control the output of evaluations. The modules communicate at a simulation resolution specified in the simulation parameters. These other modules have less impact on pedestrian movement than the program module responsible, but still display some effect. For example, with a low simulation resolution, a pedestrian could skip a very narrow area on which a pedestrian routing decision lies. The routing decision would then have no effect on the pedestrian. The simulation resolution also has an impact on pedestrians that enter escalators: with each time step of simulation resolution, only one pedestrian may enter the escalator.
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