The clogged roads you talk about is a good indication of what might happen to any large city in the US faced with sudden evacuation due to a WMD attack. I would have an alternate bugout plan that involved walking out with an A.L.I.C.E. pack, Camelback hydration system MRE's, AR-15 and Kimber .45.
Unfortunately, I believe you are right. There are still situations in the USA where folks could have to take care of themselves.