A new feature of Civil 3D 2018 are commands to resolve corridor bowties.
Bowties happens when the corridor links crosses each other, creating an overlap in the corridor.
There are 2 methods of cleaning up bowties on fixed width and variable with corridors.
For a fixed width corridor, options for clearing bowties will be found in: Settings tab > Right-click on Corridor collection > Edit Feature Settings
For variable width corridor, such as a corridor that daylights to a surface, a Clear Corridor Bowties command as been added. The command is found: select the corridor, click Corridor tab > Corridor Tools > Clear Corridor Bowties.
Here’s a video tutorial that will show examples for fixing bowties for both corridor types: