If there was any civilization left, they'd be able to pick up radio or TV or short wave or something. But they apparently haven't even tried anything like that in a while.
The President of the United States has extensive use of the airwaves in an emergency and the zombie Apocalypse didn't knock out satellites nor I'd think emergency transmitters at radio stations which IIRC by law have months of battery life to keep transmitting in an emergency.
BTW, now that I'm thinking about it - why wouldn't Rick and group us car GPS systems to navigate to safe havens instead of using maps?
Well, how fast do we think civilization would collapse? And how widespread is this apocalypse? The only place we've seen in the show is Georgia, more-or-less. Has the entire world collapsed? Or are there pockets of civilization still out there, even in the US?
I'd think that slow moving zombies would find it difficult to overtake any large military installation based on the amount of fire power they have versus the threat
Also, what about nuclear submarines, aircraft carriers and even the International space station?