(1) This is an intelligence scale for trails.
Dumb trails get lower grades, so they stay at the bottom of the list, and have to keep to the back of the room.
Smart, cool trails come in at the top of the list and get all the hot dates.
And (you guessed it) trails of different grade ratings don't hang out with each other.
Sometimes the untidy, wandering rambling lower-grade trails are mean to the smart trails. Sometimes it's the other way around.
Rarely do you see mismatched trails finding common ground, and forming friendships, but if they do they keep it secret and don't let on at crossings. Been there, right?
(2) Engineering again, for those so inclined...
At-grade trails intersect at the same level (or else they technically couldn't intersect, see). But that isn't the whole story. Never is.
Like, take a map, and it shows one road cutting across another one? Or a railway and a road intersecting? But if you go there you see that maybe one road is on a bridge over the other, or goes below the tracks. That's a grade-separated crossing.
Works with a bridge, a tunnel, stuff like that.
Mostly trails meet at a common level, but maybe a trail will cross a railway on a foot bridge, or if this is all in a city, the trail or foot path might pass under a road in a shallow tunnel. Anything to keep the two incompatible routes separated, even if it's only by grade.
Keeps you safe and you don't even have to wear a condom.
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Have extra info to add?
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Me? Recently awarded second place at this year's Doofus Awards. Hoping for third place next year.