Triskelion

As I rode the subway home today I spotted a symbol exactly like this one spray painted on a wall deep inside the train tunnel. I was only able to come across this because during the ride the train began to slowly coast to a stop in the middle of two stations, as it does from time to time. I only barely glimpsed it as it passed because the train continued on past the wall where it was drawn, stopping several feet away, and effectively obscuring my vision. Upon arriving home I scoured several of my books on occult symbology. Eventually I found it to be a variation of the Triskelion which is considered as a ward or protection from evil or malevolent forces. Among all the cramped urban graffiti painted one on top of the other on the walls of that tunnel, the Triskelion was allowed a wall all of its own. How do the urban explorers, leaving a piece of themselves behind forever splattered in paint on a wall, know to respect that very old occult marking? And what do they fear it protects them from?