The most realistic scenario on which you will have to use aikido to protect yourself on the street is tripping and falling. The best defense against cracks in a sidewalk jumping out and grabbing you is good ukemi.
Not much concern around here about being attacked by a karateka around here either.
Another thought on aikido and pacifist behavior. One thing I have really gotten to understand is that aikido is not a passive art. Just because one does not seek out a fight does not mean that one is not capable of violence.
As sensei said in class last night. If you think that you are one of the meekest and mildest people on earth you probably need to do some serious self examination.