I involved a friend (Ola J.) who helped me searching. He found an article by B.K.P. Horn that was called “The Curve of Least Energy”, which contained some pages about a spiral called the “Cornu Spiral” or “Clothoid”.
This spiral is very useful when designing roads, rails and roller-coasters. Segments of the spiral are used as transitions between for example straight and curved parts and give the smoothest possible changes in curvature.