I’m fascinated by active walking models — how unconscious patterns emerge from people’s trail-use. There are many famous examples, some anecdotal, some research-driven (c.f., here and here), but they all share the characteristic patterns, with people demonstrating walking inertia, following paths of least resistance, etc.
There, of course, some differences when walking on gradients. People tend to want to walk in zigzags, but that instinct varies against the gradient on which they are walking, as well as the nature of the zags and zigs. For example, people tend to think they’re moving faster than they are, and they take an extra step before changing direction, thus wearing the outside of corners on grades. (This is also a function of slightly less gradient at curve outer perimeters as well, so there is more than one thing going on here.)