I have lived in the middle of many evacuations. It is really a crap shoot as to which way to go. Unless you leave very early (like before most employers will release you), you are likely to get caught in traffic snarls and ride out tornados in the open. I was shocked to see I-10 (east-west) at a dead stop for many, many miles at the height of Hurricane Opal. The same was true for the north-south US Highways. The roads were clogged, cars had mechanical problems and/or ran out of fuel. There was little or no food, water, sanitation facilities or emergency responders. I can't imagine what would happen in the event of a heart attack, stroke or prenatal event. Since one rarely knows for certain where the eye will hit or how far out they will encounter tornado volleys, evacuations are crap shoots. Inland folks could be just as well off seeking shelter in a sturdy building and hoping that it can withstand a chance tornado hit. After many years of dodging these monsters, many folks feel that evacuation plans are no better than stampedes initiated by politicians at the last minute to cover their rears.
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.