Hi,
A little background on roundabouts - the feature originally came from Autodesk vehicle tracking which was a British technology company that was acquired a couple of years back... Autodesk Vehicle tracking is an add on that can be installed on top of your civil 3d software. It will simply add a new ribbon in civil 3d. Vehicle tracking is already included in the AEC collection. The reason why I am explaining this is because the same roundabout civil3d object is created when you use the tool provided in vehicle tracking but you will have more control and more modifying commands to ensure the design is just the way you intend, one of which is the setting for right or left driving behavior...
if you have the collection subscription you will difenetly benefit from installing vehicle tracking.
https://www.autodesk.com/products/vehicle-tracking/overview
Thanks
Samir Rezk
Technical Support Specialist