Here's a situation that I find myself in way too often. You're walking to work. You've stopped at what you know to be a busy intersection, because you walk it every day. You know, for instance, that even though it may look safe to cross right this second, that the right turn only signal that you can barely see is green, and a steady stream of traffic is about to come flying by.
Now imagine that while you wait, another commuter walks up behind you who obviously is not paying attention. Maybe she's talking on her cell phone, maybe she's just unfamiliar with this particular intersection. But she's about to walk into it.
What do you do?
Do you say "Hey!" or something else equally brilliant? Do you reach out a hand to hold her back? If she's about to walk right out into the street would you actually pull her out, possibly risking injury to yourself in the process?
Of course, your guess could be wrong and maybe she knows exactly what to wait for, and wasn't planning on going into the street, in which case putting your hands on her was a big mistake.
What do you do?
In general I watch such people carefully until I am convinced that they are about to do something dangerous (naturally this bar moves depending on how quickly the traffic is coming). When they've reached the point of no return I'll say, loudly enough to get their attention, "Careful!" and point at the oncoming traffic, or the green light, or whatever else will give them enough information to realize they're about to die.
Thus far it's worked, nobody has ever walked into traffic on my watch. But each time I do that, I do wonder whether I would jump out and pull somebody back if they didn't listen to me. It's a weird feeling. I don't particularly want to get hit by a car either.
And besides, every now and then you're gonna get a moron who deliberately walked in front of an oncoming car because they think they've got the right of way and are trying to be a royal ass about it. Those are the ones who I want to see get a good bump, just so I can say, "Officer, I saw the whole thing, that moron walked right in front of the oncoming car on purpose." :)