When I went to Purdue in the early 70s, the grounds crew never built sidewalks after a new building was put up. Instead, they just sat back and looked to see where the footpaths developed (figuring, as well, that college kids were too grungy in those days to worry about mud on their shoes).

Then, after seeing the footpaths which mapped for them the most efficient pathways, they then poured the sidewalks.

This illustrates the concept pretty well. Thanks!