Almost all roundabout design documents leave the alignment of the fastest path line up to the designer.
The UK standards describe it as 'the smoothest, flattest path that a vehicle can take through the entry, round the center island and through the exit (in the absence of other traffic)’.
The US standards describe it as ‘the smoothest, flattest path possible for a single vehicle, in the absence of other traffic and ignoring all lane markings, traversing through the entry, around the central island, and out the exit’.
Autodesk Civil 3D 2023 creates the path as follows:
- Create a bounding envelope from the defined curb and centerline offsets (defined in the standard).
- Fix the start and end stations (defined in the standard).
- Connect the start and end stations with a straight drive line.
- Where the drive line cuts the boundary, calculate a new station on the boundary at the point furthest away from the drive line.
- Connect the new list of stations with a new straight drive line.
- Repeat steps 4 and 5 until the drive line does not cut the boundary.
- Optimize the line to remove redundant points which might force artificially tight radii.
- Create a smooth interpolated Bezier curve through the fixed stations.