"One can imagine what it would be like with no warning if several radition (dirty) bombs were set off in Houston."
No need to evacuate entire city for one (or several) dirty bombs.
http://64.233.167.104/search?q=cache:DuYKSapzK-gJ:www.fas.org/faspir/2002/v55n2/dirtybomb.htm+nuclear+dirty+bomb+effective+area&hl=en
"Imagine that the cesium in this device was exploded in Washington, DC in a bomb using ten pounds of TNT. The initial passing of the radioactive cloud would be relatively harmless, and no one would have to evacuate immediately. However, residents of an area of about five city blocks, if they remained, would have a one-in-a-thousand chance of getting cancer."
Really? I never knew this. Can you imagine the hysteria of the media, though? These facts would totally be lost in their hype.