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Driving states in the traffic flow model according to Wiedemann

Vissim's traffic flow model is a stochastic, time step based, microscopic model that treats driver-vehicle units as basic entities.

As the model accounts for psychological aspects as well as for physiological restrictions of drivers' perception, it is called psycho-physical car-following model. It contains a psycho-physical car following model for longitudinal vehicle movement and a rule-based algorithm for lateral vehicle movement. The models deployed are based on Wiedemann's extensive research work.

• Wiedemann, R. (1974). Simulation des Straßenverkehrsflusses. Schriftenreihe des Instituts für Verkehrswesen der Universität Karlsruhe (seit 2009 KIT – Karlsruher Institut für Technologie), Heft 8
• Wiedemann, R. (1991). Modeling of RTI-Elements on multi-lane roads. In: Advanced Telematics in Road Transport edited by the Commission of the European Community, DG XIII, Brussels

Wiedemann's traffic flow model is based on the assumption that there are basically four different driving states for a driver (Traffic flow model and light signal control):

• Free driving: No influence of preceding vehicles can be observed. In this state, the driver seeks to reach and maintain his desired speed. In reality, the speed in free driving will vary due to imperfect throttle control. It will always oscillate around the desired speed.
• Approaching: Process of the driver adapting his speed to the lower speed of a preceding vehicle. While approaching, the driver decelerates, so that there is no difference in speed once he reaches the desired safety distance.
• Following: The driver follows the preceding car without consciously decelerating or accelerating. He keeps the safety distance more or less constant. However, again due to imperfect throttle control, the difference in speed oscillates around zero.
• Braking: Driver applies medium to high deceleration rates if distance to the preceding vehicle falls below the desired safety distance. This can happen if the driver of the preceding vehicle abruptly changes his speed or the driver of a third vehicle changes lanes to squeeze in between two vehicles.

For each of the four driving states, acceleration is described as a result of current speed, speed difference, distance to the preceding vehicle as well as of individual driver and vehicle characteristics.

Drivers switch from one state to another as soon as they reach a certain threshold that can be described as a function of speed difference and distance. For instance, small differences in speed can only be perceived at short distances. Whereas large differences in speed already force drivers to react at large distances.

The perception of speed differences as well as the desired speed and safety distance kept vary across the driver population.

The driving state attribute in Vissim does not specify a driving state according to Wiedemann. The attribute gives a more detailed description of the current driving state of the vehicle in the simulation on its route through the Vissim network (Driving state of a vehicle).