Always know that mobility might become reality.

The rule of thumb for any Urban dwelling survivalist is to be able to be gone from your home with all of your basic survival needs within 30-45 minutes of any major attack. 

Here is the thing. You people live in an area that will be the center point of mass chaos, unlike us rural people. The chaos will hit us eventually but it will begin where you are. For example let’s look at Katrina again. During the hurricane and the aftermath of the hurricane the government could not get to the scene to supply aid, so with lawlessness the "thugs" will prevail. They will thrive in this setting because they have no reason not to. To these people survival is not as crucial; these will be the people that come to your home and steal your TV and stereo while leaving your food behind. The problem is to get the TV and stereo they will not hesitate from using violence. 

It is not an uncommon thing to seek comfort goods when you have the mentality that you will be able to use these goods once the power grid and a since of structure returns but once these people realize that there is no return you need to already be gone!

The chaos that followed Katrina is a perfect example of what could happen in any city in the event of a natural or manmade disaster. To me it seemed like the US Govt. and all the Emergency Response agencies to actually get to New Orleans and give aid. Imagine for a moment that 10 or 20 US cities were spontaneously hit by an attack or natural disaster, do you think the government has a protocol plan for this type of emergency? I know that individual cities and states will be able to react but they will be so flustered with responsibilities an armed robbery or murder will not be as high of a priority as fighting off the culprits of the attack of sifting through rubble to find survivors. 

In other words I would not place too much trust into protection from the government or police. 

So for this reason you need to be ready to go at the drop of a dime, before you are (1) victim of a crime (2) victim of a martial law!

Some things you can do to prepare yourself to be ready are to have a simple survival pack ready to go some things you need to be sure to have are:

7-10 days worth of food and water for each person in your traveling party

Some type of defensive weapon, most preferably a gun and ammunition

Gas for your vehicle

Blankets 

Candles

A hand crank radio

Cellular phone with Solar Battery Charger

A pocket knife as well as another knife for skinning and close quarter’s protection

AND A LEAD FOOT!

If you are truly wanting to prepare for something that MIGHT never come you could have this stuff ready to go at all times in a covered trailer that you can pull behind your vehicle. Make sure this trailer is hidden or in your yard where people cannot see you loading it, I would hate for you to be targeted right away!

Another thing to do when you need to travel quickly is always make sure your vehicle is ready to go! Imagine calmly (we survivalist don't panic when the crap hits the fan) loading your family into your vehicle, hooking on the trailer, getting on the highway and next thing you know you are closer to the epicenter of the chaos and you break down because you forgot to get that tune up you've been meaning to get. 

So leaving the city because of the violence is a definite! Another reason you might have to leave is that as soon as people begin to realize that this is how it is going to be, that there is no aid coming for them, they will start looking for goods and they WILL be using any means unnecessary to get them! 

So your urban garden is nice and your urban chicken coop is a wonderful thing, but if I were you I would also have a backup plan and my back up plan would include some natural rural settings! 

However, we rural dwellers need to be ready to go too! It will not be as sudden as an urban dweller would need to be but if stuff starts moving towards your area and you are unable to defend yourself; you need to have another location that you can move to! In that case I would suggest the same materials and things to pack with you. 

I say a 7-10 day supply of food and water only in a less severe catastrophic event. If you KNOW that there will be nothing out there once you get out of the city limits then take everything you have! But you should have the 7-10 ready to go at all times! You can always add to it later if needed.