When Roundabouts in Civil 3D are created there is extra functionality over Vehicle Tracking that allows you to go construct part or all of a 3D corridor model.
AutoCAD Civil 3D 2018 can optionally create the following:
The creation of these objects when the junction initially created is controlled by the Corridor drawing settings and, once the junction has been created by the Corridor junction properties.
AutoCAD Civil 3D 2018 may also be configured so that every time the roundabout geometry is changed these alignments, profiles and corridor are automatically rebuilt.
This roundabout object is also linked to the polylines or alignments upon which it is based. So, if you create your roundabout based upon existing Civil 3D alignments, the roundabout updates automatically when you move these alignments. This functionality allows you to build AutoCAD Civil 3D 2018 roundabouts into existing corridor models and have them behave like a Civil 3D intersection object.