2.4.2 - Islands
Step Description:
1. On the Create Roundabout - Islands dialog box, set the desired options for the following settings:
- Predefined parameters to import
- Construction triangle parameters
- Splitter island parameters

2. Click the Next > button to proceed to the Markings and Signs dialog box.
AutoCAD Civil 3D 2012 User's Guide reference:
User's Guide - Dialog Box Reference - Intersections and Roundabouts Dialog Boxes - Create Roundabout - Islands Dialog Box
For more information watch the video below:
Create Roundabout (WMV)