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.

You can define boarding delay functions for passengers boarding PT vehicles.

Vissim provides functions such as Critical speed and Lateral drift speed for the simulation of critical traffic situations.

The values of the functions are displayed depending on the network settings for units (Selecting network settings for units).

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.

When a vehicle approaches a static obstacle, such as a parking lot or PT stop, it determines at each time step whether the required deceleration is already as high as the desired deceleration. If this is not yet the case, the vehicle continues to drive normally, unaffected by the obstacle. In the first time step that the required deceleration reaches the Desired deceleration, it usually exceeds the desired deceleration slightly due to the movement in the previous time step. If the Maximum Deceleration is less than the required deceleration:

• The vehicle is only able to stop a little further downstream from the intended stop position. This may be too late, especially in parking lots.

• For a public transport vehicle, the maximum deceleration (in terms of amount) can be exceeded in the last time step so that the public transport vehicle stops at the stop.

• Boarding delay: Shows the relative occupancy of a public transport vehicle in relation to the maximum capacity for a defined duration. Delays in boarding can be caused, for example, through heavy use of the train, luggage being carried, a difference in height between the platform and the train, or ticket sales by the bus driver. You select a boarding delay function for a boarding delay type (Defining boarding delay types). In the relation Doors, you assign the boarding delay type for the desired 2D/3D model segment to the respective doors (Editing doors of public transport vehicles).

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.

The upper limit for the maximum acceleration of the vehicle is the lowest of the maximum values from the maximum acceleration function, the desired acceleration function, parameter CC8 or parameter CC9 from the car following model. This means the acceleration of the vehicle is not higher than the lowest of these values. The following applies to cars only: z is added to the maximum acceleration used: z = value from the interval [0.1], normally distributed by 0.5 with a standard deviation of 0.15. The acceleration of the vehicle may be less than its maximum acceleration if other factors are involved, for example if the vehicle is very close to its desired speed or if it perceives a slower vehicle in front.

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.

Note: Make sure that the maximum value of the maximum deceleration is greater than the values of the desired deceleration. This ensures that the jerk limit is calculated, which is not always possible with a smaller or the same value.

 

Tip: Vissim provides default acceleration and deceleration functions for vehicle types typically used in Western Europe.

Functions concerning speed in curves and critical traffic situations

• Critical speed: Define curvature radii for which different critical speeds of a vehicle apply. These values are used in the following calculations:
• Lateral drift speed and lateral deviation of the vehicle due to excessive speed (Defining curve 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).