Defining curve speed

You can define monotonically increasing critical speed functions to specify different radii. With each of these radii, a critical speed applies for the vehicle. These values are used in the following calculation:

In the Vehicles in the network list, you can display the Lateral deviation (excessive speed) of the vehicle (Displaying vehicles in the network in a list).

1.  From the Base data menu, choose > Functions > Critical speed.

The list of defined objects for the base data type opens.

By default, you can edit the list (Using lists).

2.  In the list on left, on the toolbar, click the Add button .

3.  In the Name field, enter a name for the function.

The window of the function opens.

4.  Click the start point or end point of the curve whose value you want to change, hold down the mouse button and drag the point to the desired value.

5.  Right-click in the line.

6.  From the shortcut menu, choose Add new point.

A node is entered.

7.  Click the intermediate point and keep the mouse button pressed.

8.  With the mouse button pressed, drag the intermediate point to the desired position.

9.  Release the mouse button.

10.  Confirm with OK.

The function is displayed in the list on the left under Critical speed functions. In the Data points list, on the right, the minimum value, maximum value and the inserted intermediate points are displayed.

Element Description

x

Curvature radius:

  • If the curve speed function is used as a desired curve speed function: A curve with the curvature radius x leads to a target speed y of a vehicle (Selecting network settings for vehicle behavior), (Attributes of links). If the smallest defined radius Rmin is > 0 m and the vehicle sees a link intermediate point with a radius smaller than Rmin the speed defined for Rmin is used as target speed. Use Rmin > 0m as the smallest value (for example Rmin = 5m) to limit the effect of very small radii. Values for radii > 1 km are ignored. If the largest defined radius Rmax is < 1 km ist and the vehicle sees a link intermediate point with a radius > Rmax, the speed defined for Rmax is used as target speed.
  • If the curve speed function is used as critical speed function: Threshold value from which critical speed y of a vehicle can trigger a lateral deviation (critical speed), if a) the vehicle is driving on a link that traverses a 3D point and b) the Radius attribute of this point corresponds to the curvature radius x (Attributes of links). Where:
  • Vissim takes into account the last 3D point passed as well as the next downstream 3D point of the link or connector on which the vehicle travels, if the radius at both points is > 0.
  • These two points can also be the first or the last point of the link or connector.
  • If both points have a different radius, Vissim uses the smaller value.
  • If the vehicle has passed the first of the two points but cannot reach the second because its vehicle route ends upstream of this point or, in the case of dynamic assignment, the path of the vehicle ends at a parking lot upstream of this point, Vissim will still use the radius of this second point even though the vehicle will not reach it.
  • Vissim calculates a speed quotient for the function Lateral drift speed (Using lateral drift speed).

y

Speed: Speed for the given Curvature radius x.

The function value is linearly interpolated between several interpolation points.

The following applies for the lateral drift speed: The affected vehicle determines in each time step whether its maximum speed within the time step is above the critical speed. The smaller radius of the previous and next 3D point of the link are taken into account. Understeer can only occur if both 3D points have a radius > 0. The vehicle is carried out of the bend at the calculated lateral speed. If at the same time a deflection in the same direction is greater and the deflection is in the same direction, it affects the lateral movement.

The vehicle attribute Lateral deviation (overspeed) indicates the lateral deviation in the current time step (Displaying vehicles in the network in a list).

y(minimum), y(maximum)

Speed: minimum/maximum speed for the given Curvature radius x.

When used as a critical speed function: The values are ignored.

When used as a desired curve speed function: a vehicle selects a target speed between y (minimum) and y (minimum) with y as the most frequent value in a normally distributed manner.

You assign the function Critical speed as an attribute to those vehicle types you wish to impact (Attributes of vehicle types).

Superordinate topic:

Defining functions for acceleration, deceleration and speed