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Speed

Speed is used in calculations to determine the minimum radius in curves (together with superelevation) and in sight distance calculations.

Where can I find the dialog?

In the toolset, go to SFI Properties - Properties - the Show button to the right of Road Class - Speed

Using the Dialog

  • Speed limit is the standard speed limit for the road.
  • Speed addition is a general safety margin.
  • Speed profile addition is an extra safety factor that depends on the curve radius. A larger radius gives a higher speed addition. The values form a curve that is interpolated during calculations (seldom used).

During the construction of horizontal and vertical curvature, the user receives continuous information about minimum radii and clothoid values. The line element is marked in red if the requirement is exceeded.

The minimum horizontal curvature is determined based on the equilibrium of forces acting on a vehicle when driving in curves.

The figure above illustrates the minimum radius for road design.

Vertical speed and acceleration are related to driving comfort and have no safety significance.

Maximum vertical speed and axle distance are two related values. This is the vertical speed of one wheel on the vehicle relative to the other. These are used to calculate the length for building up superelevation in curves. Maximum vertical acceleration is used to calculate crest curves, so that the driver does not feel pressed down into the seat.