I can tell you from bitter experience that people don't intervene and they don't call the Police either 
My ExH slapped me in the face once in a busy shopping arcade on a Saturday. I had blood running down my face from my nose. No one stopped to help. A couple of people waggled their eyebrows in what I suppose might have been an 'are you ok?' way, but really, what could I have done. No one called the Police.
I might not intervene directly, especially if the perpetrator topped 6 foot and was built like a brick outhouse, but I have always called the Police, often quite ostentatiously. DH has intervened when we've been out.
four no question is ever really idiotic.
Sabrina that video was horrific to watch, it left me in tears.
I knew there was something hinky about the Mankind video, now I've read the We Killed The Mammoth blog and watched it again, it's actually quite sneaky.