Both are possibilities. My contention is foremost they'd just be thinking of getting out of the war zone. I'm not sure if either the source of food or water will give much weight. Probably they have friends or relatives outside the city they plan on getting to. Hoping that place will be better then where they currently are..
Kinda like the mass exodus from Manhattan post 9/11. Many had no bigger plan then to not be there anymore. Anyplace else had to be better. Same thing happens in war torn areas. Folks pack up what they can and just follow the crowd. usually they all end up together at some refugee type camp on a border. The neighboring country not allowing them to cross.
Even the Hoovervilles, or Obamaburbs (tent cities that have popped up) don't seem to follow much logic other then they're not being chased off that ground.
My guess is that, initially, most people *won't* leave a city unless things become untenable. They'll wait for the government to do something first; only when pain sets in, or thirst, or hunger, will they consider moving.
I can't imagine having an EMP, say, and suddenly the roads are clogged with people walking out of the city.
People will be confused at first, seeking information, looking for help. I'd doubt that any more than 1 or 2 in a hundred would have their wits about them enough to recognize EMP for what it was, and to know what to do.
That 1-2 percent would be, of course, us.