Civil 3D 2018 Bow Tie Cleanup
The Civil 3D Bow Tie Cleanup for Variable Width Corridors
Jeff Bartels of Autodesk covers the basics the manual Bow Tie Correction Tools in Civil 3D 2018 for variable width Corridors.
From the Jeff Bartels YouTube Channel.
and the Civil Imersion blog.
For fixed width corridors you can resolve bow ties automatically by changing the build properties of the Corridor itself. That functionality is also included in Civil 3D 2017 with latest Updates and Enhancements installed.