It seems to me that the Department of Homeland Security's "National Threat Level" is flawed from the start. Why? This is a vast country, and its actions and citizenry are spread out across the globe. What can manifest as a threat in one area is no threat whatsoever in others.
The use of a single metric to describe conditions across a whole nation's activities is deceptive and unnecessary fear-mongering. I suggest to you that the average small-town dweller in Idaho need not worry about catastrophic acts of terrorism.
A more realistic approach to the problem would include the acknowledgement that in a country of this size, terrorist acts are likely to affect a relatively small number of high profile target areas. These areas would include, obviously, New York and Washington, as examples.
It is therefore substantially more rational to target warnings about terrorism to those areas that are truly at risk, rather than alarming the population in general.