If you are running AutoCAD Civil 3D 2018 on a supported platform, a corridor tab is available. This defines how the 3D model of the roundabout is built and updated.
Click Corridor in the left hand pane. The Corridor page is displayed.
The checkboxes in the upper section of the page control how much of the 3D model AutoCAD Civil 3D 2018 creates and updates.
Select Create Alignments to create horizontal alignments in your CAD system.
Select Update Alignments if you want AutoCAD Civil 3D 2018 to update the alignments automatically when the geometry changes. This option is only available if AutoCAD Civil 3D 2018 created the original alignments, i.e. if Create Alignments is selected
Select Create Profiles to create vertical profiles along the horizontal alignments.
Select Update Profiles if you want AutoCAD Civil 3D 2018 to update the profiles automatically when the geometry changes. This option is only available if AutoCAD Civil 3D 2018 created the original profiles, i.e. if Create Profiles is selected
Select Create Corridor to create a 3D corridor model using default assemblies.
Select Update Corridor if you want AutoCAD Civil 3D 2018 to update the corridor automatically when the geometry changes. This option is only available if AutoCAD Civil 3D 2018 created the original corridor, i.e. if Create Corridor is selected
Select Add outside curb and ground cut and fill assembly to add a curb on the outside of the corridor with a small gap on the curb away from the road and then a cut and fill section.
If Require Existing Surface is selected, the Corridor is only created if the Roundabout has its existing ground property set.
Create Corridor Top Surface
Regardless of any of the settings, you can force a rebuild of all the selected elements by clicking Rebuild Now.