Racial profiling

I had an odd moment today which has made me re-assess my prejudices.
I was walking down Shattuck at 10pm with Cat Stevens in my ears when I suddenly became aware of this big,young, black guy on my shoulder. He was jogging aimlessly in the same direction I was headed. I had sub-consciously noticed him walking aimlessly at the crossing but he seemed harmless. However, in that instant where I noticed him, I visibly flinched, frightened. Obviously, this guy noticed it. He gave me a questioning nod and started talking to me. I hid behind my earphones until politeness prevailed and I removed an earpiece to listen to him.
Guy: "Hi!"
Me: "Hi!"
Guy: "Do you know me?"
Me: "No"
Guy: "Cool". We high five with our fists (if you have seen American movies, you know what I mean..Can't explain it)
This conversation took place while he was walking alongside me. I was not sure where all this was leading. Embarassed to have displayed my fright, I obviously did not wish to be rude or provoke an obvious comment and continued making conversation.
Guy: "Do you two dollars, man?"
Me: " Sorry mate, no cash on me"
Guy: "&^*% off." veers away from me and continues to amble away in search of his next victim.
All this happened within the space of 1 minute, although it seems longer.

Anyway, it set me thinking how I had sensed danger when I saw another person behind me. I don't think I have ever had such an experience in the UK but the entire atmosphere in the US of crime and insecurity alongwith the Berkeley homeless population seems to have set me on edge. I may be over-exaggerating the episode, but a primitive,subconscious process perceived this man as a threat and I began to wonder why? What sort of memories and information does my innate defense system process and in what alogarithm to alert me to an uncomfortable situation?

My analysis of the event:
1. Possible homeless man(call him subject henceforth) at crossing. I saw another man walking in opposite direction from subject ( I do not know if the subject caused man to walk away, but I made that assumption)

2. MAN not Woman. Women are less of a danger signal
3. Black Man not White Man not Indian not Chinese - I suspect, and this is what scares me the most. I think in my head and subconsciously, I racially discriminate. I am sure I did this even in the UK, although in Oxford I did not have much opportunity to feel threatened or uncomfortable.
4. Big Man

I rate demographic groups as dangerous in this order:

Big Black street young guy > Black homeless old drunk man > Big White homeless man > White drunk man > Drunk Person

In the UK, I had a single danger rating:
Big drunk Man

I am terrified that I am in a sense racially profiling people. It is wrong and I am going to have to work hard to eradicate it. I have in a sense almost stereotyped racial and ethnic groups and my instincts reflect that stereotyping. An education and civil upbringing allows me to suppress these baser prejudices with rational equality but extreme circumstances might force me to behave instinctively not after rational evaluation of the situation. then what??

“Racism is man's gravest threat to man - the maximum of hatred for a minimum of reason.”