Thank you very much for your perspective, but please let me clarify this point: my story doesn't necessarily have to portrait a total breakdown, a mass evacuation or scenes of panic and violence. As I said in my first post, this would be only an episode in the whole narrative. I'm not trying to write a survival or apocalyptic novel. What I'm asking is what do you think would realistically happen given the circumstances I described (a terrorist attack followed by a power outage).
The decision of B to bug out immediately doesn't necessarily imply a critical situation (i.e. a total evacuation of the city). I'm still working on this, but B may have been urged in bugging out by some foreknowlegde of A that things may be getting worse in the following hours, not only in NYC but other US locations as well.

Thanks to everyone for your suggestions, please keep up!