A fair number of members here live in cities. My SHTF scenerio is not bio-agents or nukes or chemical. I worry about what would happen if they took out our power grid. You start to think about dependent our cities are on electricity, and you'll know what I mean.
I live in the Bay Area, and it was weird enough with rolling blackouts last winter. Now imagine that power is completely out for a week or two. Our home is at the top of a hill, no water. Traffic lights, food storage, hospitals etc. Two and a half years ago a repair crew accidentally knocked out power to San Francisco for over 8 hours. I was stuck in a train under the bay for 2 of them. I had to walk 2 1/2 miles to my parents place, because I couldn't go home, couldn't call my fiance. Luckily no civil disturbance! Most yuppies were panicked, they couldn't get espressos! But that was a minor inconvenience.
My neighbor is a line manager with Pacific Gas & Electricity. We talked about this, and he knows it could be trouble. There are very few power lines coming in, and they all run through some remote areas.
I have a rather low expectation of the cohesiveness of the community when most people are voting to get more goodies from government. These people have no ability to function in civilized manner if they have grown so accustomed to having everything handed to them.
Cut power to the cities, and watch how quickly our civilization breaks down. I really want to leave the Bay Area, now just need to find a job paying $100K in some wooded, quiet area.