James, you said:
" Sounds nuts to me. The error you calculated assumes the course has no concave sections. In real life the errors would be larger. It's not just the outer corners of the course. Inner curves, or concave sections, would also shorten the distance. On a wiggly road, and even in Florida we have many, the SPR would be much shorter."
I gave the answer to the problem Seutter proposed, not to the problem he didn't propose.
Propose a different problem, get a different answer. The more the curvature, the more will be the difference between the SPR and the middle-of-the-road route.