Just adding to what Hussar said: if I succed on the Tracking roll, then the upshot of following the trail of dropped items etc is that I find my quarry; if I fail, then the upshot is that the items lead me astray.
Also, on this issue of tracking over cobblestones: I live in a suburb with many cobbled lanes. Cobblestones hold mud (and you can clean your shoe by dragging it across a cobblestone); plants grow in the cracks, and those plants can be knocked over or torn up by passing feet; puddles form in unevenness on the stones, and those puddles can be splashed; if it stops raining, water from puddles can be stepped in, leading to wet footprints on the stones; etc.
I have no skill at tracking, but I don't see why cobblestones are so much harder to track over than, say, a relatively featureless grassy plain. Whereas the idea that the sound of footsteps, short of an army or a herd of cattle, might travel 10s of miles through the ground seems rather fanciful to me.
The point is, the DC is set high (shy of an actual RAW rule for the DC) because it is a hard to track on surface, and being a road, it might have some significant amount of traffic. If the roll is made, the DM can describe it however he wants, but I still wouldn't make it a low DC.
A featureless grassy plain would tend to have very few travelers all traveling the exact same route. There might be a few paths, but then again, those paths should often be dirt/mud, not stone. The harder the surface and the more travelers on it, the harder it should be to track.
As for listening for horses galloping miles away, American Indians reported being able to do that. I don't think anyone is claiming that someone could hear normal footsteps at that range.