This question comes up fairly often so I thought I would create a post that addresses the issue
If you create a Vertical Design, the default properties of a Vertical Design are that when VPIs are created in lines either through applying the Vertical PI rule or when you apply e.g. a Cross Slope rule from a line that incorporates vertical curves to a 2D line that the vertical curves created are Vertical Arcs.
You can change the default behavior of a Vertical Design by changing the properties of the Vertical Design. Find the Vertical Design in the Project Explorer and select it. review its properties. There is a property called Vertical Curve and it will likely be set to Arc. Change the curve type to Parabola if you want to use Symmetrical Parabolic Vertical Curves.
If you take a straight line alignment, starting at Station 0 and ending at station 500, and in the middle at Station 250 you place a vertical curve with VPI Elevation of 105 and Curve Length 200 and at station 0 and 500 you have Grade Breaks at elevation 100. Then you have offset lines e.g. for Edge of Pavement at 12’ offsets. If you now create a Vertical Design using the three lines (center line alignment and Edge of Pavement x2, and apply a simple cross slope of -2% to the two EOPs from the centerline. In this case the EOPs are elevated and the lines will have a combination of straight segments and parabolic vertical curves. This combinations will collectively replicate the vertical curve of the centerline alignment, and will be computed using the Surface Settings (Sampling Distance, Horizontal and Vertical Tolerance for the Breakline Approximation Parameters as set in the Surface Settings of the Vertical Design, and also the Line geometry fitting tolerance as defined in the Other settings of the Vertical Design.
The offset lines are computed using least squares methods, and will never be identical to the main line geometry, because the vertical design has to factor in the shorter lengths on the inside of the curve and longer length on the outside of the curves, and also the fact that multiple vertical design rules may be affecting the final vertical geometry, however in “perfect conditions” with high accuracy settings, the end result will closely match the source alignment vertical geometry.
The beauty of the Vertical Design method is that the resulting linestrings of the EOPs retain their original CAD geometry (Arcs and Straights) and the VPIs on the vertical have combinations of Grade Breaks and Vertical Curves (Arcs or Parabolas) and are not chorded in any way.
These linestrings can then be converted into Alignments and can be used e.g. for stringless paving applications.