Is it possible to buffer a line feature and then break it down into smaller polygons by clipping parallel to the line endpoints ( see screen capture below taken from Paving the Road Ahead with ArcGIS and AI (esri.com)) or to create thiessen polygons using endpoints and ensuring that they follow the line segments?
Please check if any of these options work for you.
One way is to create Buffer with End_Type as "Flat".
1. Create Buffer with End_Type as "Round".
2. Using the line Feature Class, execute Minimum Bounding Geometry , choose a suitable bounding Geometry Type (Rectangle by Width).
3. Clip the Buffer Boundary using the MBG boundary.