Convert 3D Polyline to 3D Alignment Object

Does it just need stripping the Z that is easy. Then make an alignment.

To take a normal 2dpline and make an alignment object no user interaction, you need to know how that is done from a programming perspective, In CIV3D there was sample code of how to do it, with lisp, there may be some one who has done the make an alignment code. Getting inside the civil database can be easy or very difficult.

Its not a simple task unless you can just use a command mode method, and maybe pass it the (entlast) variable name that is the 2dpoly, Did you try

PS 3dpoly to 2dpoly just google.

Where I use to work and they still do use CSD it is a complete civil package, it has years of development and is ahead of Bricscad CIVIL in many ways. Yes it has a cost but the extra features will pay for them selves saving you time. Also has drainage and sewerage.

The intersection tools are great time savers including roundabout designs.

Google say Civil site design youtube subject where subject is say "Make alignments" there should be Road design part 1 which should show how to make alignments and make simple road design producing Long/plan & Cross sections.

Actually, there's a sample in Brx I converted to Python

from pyrx import Ap, Db, Ed, Ge, Gs, Cv


@Ap.Command()
def doit() -> None:
    # Get the current drawing database
    db = Db.curDb()

    # Prompt user to select a 3D Polyline
    ps, id, pnt = Ed.Editor.entSel("\nSelect Polyline: ", Db.Polyline3d.desc())
    if ps != Ed.PromptStatus.eOk:
        raise RuntimeError("Oops {}:".format(ps))

    # Open the selected polyline
    pline = Db.Polyline3d(id)
    plpnts3d = pline.toPoint3dList()

    # Make sure the polyline has enough points
    if len(plpnts3d) < 2:
        raise RuntimeError("Invalid input")

    # Create a new, empty Civil horizontal alignment object
    hAlignment = Cv.CvDbHAlignment()
    
    # Extract the underlying geometry curve data from the polyline
    composite = pline.getAcGeCurve()

    # Loop through each segment, convert to 2D, and add to the alignment
    for seg in [Ge.LineSeg3d.cast(s) for s in composite.getCurveList()]:
        hAlignment.addLineFixed(seg.startPoint().to2d(), seg.endPoint().to2d())

    # Verify that segments were successfully added
    if hAlignment.elementCount() == 0:
        raise RuntimeError("Failed")

    # Save the new alignment into the drawing's Model Space
    db.addToModelspace(hAlignment)

Having done years of road designing, you may get used to using the python option to make an alignment so would it be better to select 2d or 3d pline and just check which object you have ?