One of my calculus classes in college involved calculating the required spline for a curve of a theoretical portion of interstate in order to meet the interstate requirements/regulations. It was pretty interesting.
Don't quote me on this since I don't remember why I believe it, but I think it's part of the legally mandated design requirements for the US interstate system.
No I'm sure there are requirements, but sometimes there are also space or budget constraints (I assume) since some curves are definitely safer and easier than others for the same highway (I-95 for me).
