Roundabout Standard Editor: Arm Geometry: Approach

Expand Arm Geometry and click on the Approach tab.

The Design Speed is the design speed of the section of road leading up to the roundabout.

The Central Gap Width is the distance between the offside definition lines of the approaching lanes and the departing lanes.

The Deviation Offset defines the section of road that follows the original alignment before horizontal deflection is applied.

Deflection, (or center line offset) is the distance that the alignment is adjusted to the left or right of the zero deflection point. Zero deflection occurs when the alignment meets the ICD on a bearing that passed through the center of the roundel. Deflection is used to adjust drivers speed.

The Arm Base Alignment Curve Type provides two calculation modes for the adjusted alignment approaching the roundabout.

• With Arc mode, a tangential arc is created off the existing road alignment arcing to the ICD.
  • The Final Straight Section defines a straight section from the end of the adjusted alignment to the roundabout.
  • The Transition Arc Radius defines the transition radius from the adjusted alignment to the straight section
• In Spline curve mode, a spline is used to the same transition.

The Approaching Lanes is the number of lanes of traffic approaching the roundabout and would normally be less than or equal to the number of circulatory lanes.

The Approaching Lane Width is measured between definition lines. On a road with no center gap the offside definition line is the centerline.

The Approaching Crown Line Start Offset is the distance back from the ICD at which the crown line starts to deviate from the approaching lanes offside definition line.

The Approaching Crown Line Offset at Entry defines the position of the crown line at entry to the roundabout. It is measured as a percentage of the entry width from the offside definition line so 0% would be on the offside definition line and 100% would be on the nearside definition line.

The Departing Lanes is the number of lanes leaving the roundabout onto this road.

The Departing Lane Width is measured between definition lines. On a road with no center gap the offside definition line is the centerline.

The Departing Crown Line Start Offset is the distance back from the ICD at which the crown line starts to deviate from the departing lanes offside definition line.

The Departing Crown Line Offset at Exit defines the position of the crown line at exit from the roundabout. It is measured as a percentage of the exit width from the offside definition line so 0% would be on the offside definition line and 100% would be on the nearside definition line.

In all cases the Max and Min values represent the limiting values from the adopted design standard and the Default value will be used when the roundabout is first placed. A default value MUST be provided but Max and Min may be blank if there is no limiting value.