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Defining functions for acceleration, deceleration and speed

To account for differences in the driving behavior of several drivers and different vehicle properties during acceleration and deceleration, Vissim uses functions instead of individual acceleration or deceleration data. Vissim provides speed functions for the simulation of critical traffic situations.

Acceleration and deceleration functions

Acceleration and deceleration are functions of the current speed. Thereby it is taken into account that combustion engines reach their maximum acceleration at lower speeds, and AC motors of trams and trains constantly accelerate over a large speed range.

In Vissim, the functions for acceleration and deceleration are displayed in curves:

• Maximum acceleration: max. acceleration technically possible. It is used to keep a certain speed on slopes, i.e. when stronger acceleration is required. The maximum acceleration is automatically adjusted for up and down gradients of links (Stochastic distribution of values for maximum acceleration and deceleration):
• by -0.1 m/s² per gradient percent incline
• by 0.1 m/s² per gradient percent downgrade

• Desired acceleration: used in all situations, in which maximum acceleration is not required.

• Maximum deceleration: max. deceleration technically possible. As deceleration values have a negative algebraic sign, the maximum deceleration is the smallest acceleration value. Not even the desired deceleration can fall below it. Example: If the maximum deceleration is -5 m/s², the desired deceleration cannot be - 6m/s². The maximum deceleration is automatically adjusted for up and down gradients of links and connectors:
• by -0.1 m/s² per gradient percent incline
• by 0.1 m/s² per gradient percent downgrade

• Desired deceleration: Is used as the upper bound of deceleration in the following cases. Thereby maximum deceleration is not exceeded.
• based on a desired speed decision
• when approaching a red light
• when closing up to a preceding vehicle, e.g. during stop-and-go traffic
• in case of insufficient side clearance when overtaking on the same lane
• when approaching an emergency stop on connectors of routes
• for co-operative braking. Thereby 50% of the vehicle´s desired deceleration are used as the max. reasonable deceleration to decide whether an indicating vehicle may change from the neighboring lane to the vehicle´s lane.

You can assign acceleration and deceleration functions to the vehicle types of your choice. In all other situations, the parameters of the car-following model are relevant.

Desired acceleration, maximum acceleration, desired deceleration and maximum deceleration of a vehicle, driving at a certain speed, lie within a certain range between a maximum and a minimum value. For each of these four functions, you can show the maximum-minimum range in a graph for the median and limiting graphs for the upper and lower threshold values (Defining acceleration and deceleration functions). The limiting graphs define the bandwidth. Red circles in the median graph show intermediate points that allow you to edit the median course. The limiting graphs show the intermediate points in green.

Modifying data points during a simulation run is possible only via the COM method ReplaceAll, which replaces all existing data points of the function with those included in the command call of the method. Individual data points cannot be changed during a simulation run.

Speed-controlling functions

Vissim provides speed functions for the simulation of critical traffic situations:

• Critical speed: Define radii for which different critical speeds of a vehicle apply. These values are used to calculate the lateral drift speed and the lateral deviation of the vehicle due to excessive speed. (Defining critical speed).
• Lateral drift speed: Calculate lateral offset Lateral deviation (excessive speed) for the vehicle whose lateral position changes due to excessive speed (Using lateral drift speed).