Back in the days before the PCV system concrete highway lanes would sometimes darken in the middle due to oily discharge from the road draft tubes.
I would also add to the slickness on a rainy day.
We will always have a lot of oil pollution from commercial trucks, especially company fleet trucks. When it comes to how companies maintain their fleet of commercial trucks, I’ve heard too many company mechanics say, “That’s not leaking; it’s just seeping. Seepage is not a leak.”