The issue is with surfaces like crushed limestone which appear monolithic but can, depending on temperature and dampness, be either very hard or very soft, so the accuracy of the calibrated bicycle method depends on the condition of the path when you measured it. Not to mention that conditions across the path can be very different from one place to another- so even setting up a calibration course on it might very well not provide an accurate calibration.
At least around Chicago, crushed limestone is the preferred surface for recreational paths that aren't paved with asphalt or concrete. Unless it's pretty wet or pretty dry it's pretty hard and I would suspect bicycle measurements are pretty true. And if I issued an edict that any course that contained a crushed limestone segment had to have that portion of the course measured with a steel tape, I can't imagine what the reaction would be.