If you had to think about it, you were not ready to step in which may end up getting yourself or the woman killed.
Sometimes Non-Action is the correction Action, but you have to know that the decision was the right one, and you don’t seem to know that at this time.
You cannot be second-guessing your decision in these matters at the time they are happening, you MUST assess the situation and act immediately based on what you seen and heard. If you cannot, and it was obvious you could not, then you were not ready to act. You would have been more of a danger to yourself and to her. Your non-action was the only choice you could make.
Seems you are also feeling a bit guilty for how you handled it. This is something you will need to address within. But understand, unless you all of a sudden became a super hero, you will probably be suffering some guilt. Don’t let it chew you up. Address the issue and learn from it, its how we grow!
My only question is did the truck leave with the woman? If so, you did not follow with 911 on the line?
Added: 15 to 20 years ago in downtown Detroit, i was in a McDonalds I think if I recall it was the RenCen. A woman came in and jumped the counter and clobbered a worker with a wine bottle knocking the out! The first woman jumped back over the counter and ran in-between my wife and myself. I just stood there.............I had not expected such a thing to happen and not one person ran after the attacker............Today I hope my heightened awareness would allow me to intervene if another such attack happened. I still think about that day and feel guilty that I did nothing, but at that time in my life, I was not in any position to protect myself let alone anyone else.