I have never been seriously attacked by someone I know. I believe in many ways the US is actually less prone to random violence than the UK. I've had about 5 major altercations (where serious damage was likely) in my life - mostly between the ages of 20-27. All of them except 1 was in areas where alcohol was involved (the other, I was travelling). I think Aleksey is correct about certain areas being more prone to violence.
However I also believe the violence done to men and women is completely different. I am suprised in Ireland (North and South) with the number of domestic abuse cases I hear - and I think much of this is also due to alcohol.
For men I think most violence is about egos and being macho (or race). For women I think it is either very random (as in above case), domestic (usually partners) or rape related. Pretty much it is about sex or power. In some cases it is also related to crime (stealing), but I actually think this is rarer.
And Brian, your story saddens me as well - unfortunately psychological studies have shown that in such situations (especially in crowds) if no single person helps, the others are unlikely to either (type of group behaviour). That is why I really make a personal effort to help those in distress (though it did get me into three of the above mentioned fights). However, in my mind, you know you're going to die, your family are going to die, and everyone eventually will die so its not whether you live, it is how you live that counts. (Also - if you get involved, people will follow your lead).