Spiral-curve-spiral 180 degree turn

[Maverick91]

I have two lines in an alignment, parallel to each other, 400 feet apart. I need to connect them with a spiral-curve-spiral turn. How can I do this?

[remi678731]

Wow I'll be watching to see if anyone has an answer better than mine. Had to do that had to either put a short tangent in the middle of the radius, or spiral to curve to tangent to curve to spiral not pretty on the plans but drivers will never know it, or make the alignments not quite parallel doesn't take much turn 179.9 degrees instead of 180.

[Maverick91]

Wow I'll be watching to see if anyone has an answer better than mine. Had to do that had to either put a short tangent in the middle of the radius, or spiral to curve to tangent to curve to spiral not pretty on the plans but drivers will never know it, or make the alignments not quite parallel doesn't take much turn 179.9 degrees instead of 180.

Yup. So far I've done the same, with a spiral in, a curve, and a zero-length spiral out to 90 degrees. Then a zero-length spiral in, a curve, and a spiral out. All with a short tangent in the middle. It seem C3d wants a PI, which doesn't exist when the lines are parallel.