Expand Roundel Geometry and click on the Central Island tab.
The Inscribed Circle Diameter is the diameter of the definition line that represents the outer limit of the outer kerb of the roundabout. Note that if a kerb offset has been defined then the actual kerb will coincide with the definition line.
The Centre Island Diameter is the diameter of the island around which the vehicles turn. It does NOT include the apron or overrun area.
The Apron Diameter is the diameter of the overrun area. This is the offside limit of turning for normal vehicles.
The Apron Width is the width of the vehicle overrun area. It is Apron Diameter minus the Centre Island Diameter divided by 2.
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.