IMHO we're talking about two different kinds of ‘prejudice’ here. If I avoid a specific area of town that has a high crime rate, and has a specific demographic, I'm not ‘prejudging’ the individuals who live there, but am prejudging the situation, the area, and the context, on the basis of previous events (crimes) that occurred in that area. I certainly don't think that everyone who lives there is a bad person, but I know that I'm at higher risk in that neighborhood.
There are areas of Europe (for instance some areas with high Camorra activity in the Naples area) that people try to avoid. Is that prejudice against Italians? No. It's not ‘prejudging’ individuals or groups, it's decision making based on past events and statistics.
If I avoid going into a ‘biker bar’ to have a beer, it's not because I think all people who ride motorcycles are dangerous or ‘bad’, but it's because statistically it is more likely that I will be exposed to trouble or violence in that establishment. It's not really individual prejudging.
Appearances do mean something, which is why there is mimicry in nature such that some snakes, plants, and insects that are not poisonous look like those that are.
So, avoiding a guy with a shaved head, tongue studs, and multiple tattoos is not because I am prejudging that person as an individual. It's because there is a reasonable concern based on societal experience that this appearance is associated with a higher likelihood of aggression.
I suspect no self-respecting Euro would go into a Roma neighborhood.