On our secondary roads here, in the middle of the lane, there is a noticeable (light colored pavement) brown line on many of the roads, obviously made by cages, leaving leaking oil and so forth.
We who ride, purposely stay to the left or right of that strip of oil residue because of it's presence and what it can potentially mean to our traction, and to the possibility of it getting on our tires, and spraying up to our bikes, and to us.
That said; I just happened to notice yesterday, that it seems that on the uphill side of hills on these roads, the stains & residue left behind, are darker, more pronounced; thicker than on the level or downhill portions of the same roads.
It spoke to (I thought) the cages (and even bikes) working harder on the uphill portion of those roads, and also to the importance of paying more attention to avoiding this residue, on the uphill, since more traction and stability would be important while we travel up hills, and in general.
It was sort of meant to be a "heads up" to remind inexperienced new riders, about another typical danger out there.
-Soupy