Roundabout Properties: Arm: Approach

Expand an arm and click Approach in the left-hand pane. The Approach page is displayed.

The Alignment Offset defines an offset from the selected defining road alignment and the centre line of the approach arm.

The Design Speed can affect the turning capabilities of the vehicle if you have limited turning by lock-to-lock time. At higher speeds, vehicles will not be able to negotiate tight turns.

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 centre 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 centre of the roundel. Deflection is used to adjust driver's speed.

The Physical Deflection Range Min and Max values are the actual physical max/min that the geometry of the roundabout will allow you to use.

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.

Select the number of Approaching Lanes and Departing Lanes from the drop-down list.

The Approaching Lane Width and Departing Lane Width are measured between definition lines. On a road with no centre gap, the offside definition line is the centreline.

In all cases the Max and Min values represent the limiting values from the adopted design standard.