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Introduction to VISSIMOregon Institute of Technology

Traffic Engineering Laboratory

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V I S S I M

The name is derived from the German words “Verkehr In Städten - SIMulationsmodell”

Approximate translation =

Traffic in cities - simulation model Produced by PTV

Planung Transport Verkehr, Karlsruhe, Germany

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V I S S I M

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What is VISSIM?

VISSIM is a microscopic, time step, and behavior based traffic simulation computer program

VISSIM models urban traffic and public transit operations

VISSIM can analyze traffic (cars, trucks, pedestrians) and transit (buses, trains, trams) operations under constraints such as lane configuration, traffic composition, traffic signals, transit stops, etc.

VISSIM has robust pedestrian modeling capabilities (particularly in V5.1)

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What is VISSIM?

VISSIM consists internally of three different parts: Traffic flow model Traffic control (signal light) model Data analysis package

The traffic flow model generates an online visualization of traffic operations

The traffic control model generates the indications (i.e. green, red, amber) which is “exported” to the traffic flow model

The data analysis package creates offline output files containing statistical data such as travel times and queue lengths

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Model Interaction

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Traffic Flow Model

Developed by Rainer Wiedemann in 1974 at Karlsruhe University, updated 1999

Car-following model that considers physical and psychological aspects of the drivers

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Traffic Flow Model

Discrete, stochastic, time step based, microscopic model with driver-vehicle-units as single entities.

Contains a psycho-physical car following model for longitudinal vehicle movement and a rule-based algorithm for lateral movements.

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Traffic Flow Model

The basic idea of the Wiedemann model is the assumption that a driver can be in one of four driving modes: Free driving Approaching Following Braking

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Free Driving

No influence of preceding vehicles observable

The driver seeks to reach and maintain a certain speed, her individually desired speed.

In real life, and in VISSIM, the speed in free driving cannot be kept constant, but oscillates around the desired speed due to imperfect throttle control.

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Approaching

The process of adapting the driver’s own speed to the lower speed of a preceding vehicle.

While approaching, a driver applies a deceleration so that the speed difference of the two vehicles is zero in the moment he reaches his desired safety distance.

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Following

The driver follows the preceding car without any conscious acceleration or deceleration.

He keeps the safety distance more or less constant, but again due to imperfect throttle control and imperfect estimation the speed difference oscillates around zero.

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Braking

The application of medium to high deceleration rates if the distance falls below the desired safety distance.

This can happen if the preceding car changes speed abruptly, of if a third car changes lanes in front of the observed driver.

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Traffic Flow Model

For each mode, the acceleration is described as a result of speed, speed difference, distance and the individual characteristics of driver and vehicle.

The driver switches from one mode to another as soon as he reaches a certain threshold that can be expressed as a combination of speed difference and distance.

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Traffic Flow Model

For example, a small speed difference can only be realized in small distances, whereas large speed differences force approaching drivers to react much earlier.

The ability to perceive speed differences and to estimate distances varies among the driver population, as well as the desired speeds and safety distances.

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?Psycho-Physical?

Because of the combination of psychological aspects and physiological restrictions of the driver’s perception, the model is called a psycho-physical car-following model.

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Car following model

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How does a 1974 model simulate 2009 drivers?

The model has been calibrated through multiple field measurements at the Technical University of Karlsruhe, Germany.

Periodical field measurements and their resulting updates of model parameters ensure that changes in driver behavior and vehicle improvements are accounted for.

Latest revisions in early 2009 (V 5.1)

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Beyond car following…

VISSIM’s traffic simulator not only allows drivers on multiple lane roadways to react to preceding vehicles (2 by default), but also neighboring vehicles on the adjacent travel lanes are taken into account.

Furthermore, approaching a traffic signal results in a higher alertness for drivers at a distance of 100 meters in front of the stop line.

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Beyond car following…

VISSIM simulates the traffic flow by moving “driver-vehicle-units” through a network.

Every driver with his specific behavior characteristics is assigned to a vehicle with its own characteristics.

As a consequence, the driver’s behavior corresponds to the technical capabilities of his vehicle

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Beyond car following…

Attributes characterizing each driver vehicle- unit can be discriminated into three categories: Vehicle specifications Driver-vehicle behavior Interaction among vehicles

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Beyond car following…

Technical specifications of the vehicle, e.g. Length Maximum speed Potential acceleration Actual position in the network Actual speed and acceleration

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Beyond car following…

Behavior of driver-vehicle-unit, e.g. Psycho-physical sensitivity thresholds of the

driver (ability to estimate, aggressiveness) Memory of driver Acceleration based on current speed and

driver’s desired speed

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Beyond car following…

Interdependence of driver-vehicle-units, e.g. Reference to leading and following vehicles on

own and adjacent travel lanes Reference to current link and next intersection Reference to next traffic signal