I've lived in the middle of Washington, D.C. for 25 years so have given this a lot of thought -- especially when stuck in rush hour traffic. Traffic is awful here on a good day. Morning and evening rush hour are each now 3 to 4 hours long.
You don't know where you'll be (or your family) when something happens -- home, work, in transit -- so try to plan for each of those scenarios.
There's something I purchased in the past year that I highly recommend for city life, period, but especially as part of your preparedness: Burley Travoy.
It's a bike "trailer" and hand-cart. http://www.burley.com/page_12208/travoy.htmlI'd looked at them for a couple years (they were new in 2010) and finally purchased one when we got a grocery store within walking distance in my neighborhood. I love the Travoy and use it not just for groceries. You could put your backpack on it or perhaps a child if they were too tired to walk. It'd be easy to carry down several flights of stairs.
Burley sells a number of bags designed for it, as well as cargo straps to secure anything you put on it.
It folds up very compactly (the wheels pop off with a single button). Very well designed and constructed. Not cheap, but there's literally nothing else like it available that I am aware of.
So I love it for everyday urban life and it would be priceless in an emergency whether it was an evacuation situation (foot or bike) or if gasoline became scarce and I was staying in the city.
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