[mattthebaker]

If I had to estimate "common and noticeable" sightings based on our old office near twitter and my experiences:

Homelessness -- multiple per block

Excrement -- every other block

Public drug use (non pot) -- every other day

Public urination -- once per week

Public defecation -- once per month

I was assaulted at the street corner near our office once in the 1.5 years we were there.

My guess is that the homeless population is just too large to manage any reasonable degree of cleanliness.