Sometimes it doesn't matter what markers and warnings are provided.
Back when I was at the BT satellite station, there was one time when they had outside contractors in to dig foundations and other trench work for one of the new dishes and associated buildings.
One of the diggers was stopped just in time to avoid his shovel going through one of the site's 11kV distribution cables (fortunately one that wasn't energized at the time, but he wasn't to know that).
When hauled in front of the manager and asked why he didn't stop when he hit the huge
marker tape with "DANGER High-voltage cable below" emblazoned in gigantic letters along it, his reply was, apparently:
"Oh, yes, I saw that. I didn't think it was important."