A lot of people are underestimating the current political climate and I think it's alarming that the steps our government is taking to prevent large-scale attacks are so costly and yet largely unproven/inaffective
The problem is that people don't trust the government. And they don't trust the police and the local authorities either.
The other thing is that people don't understand how to deal with radiation.
These will be major factors influencing the way any dirty bomb attack is dealt with. Public panic will be a major issue.
"Stay calm, stay where you are, stay indoors, listen to the radio" is actually sound advice. But a lot of people will ignore it, because they think it means "f*** you, you're not important".