The problem is that the color-coded threat levels were vague and long-term, and didn't correspond to useful actions people can take. Compare the color-coded threat levels with the U.S. military's DEFCON system. At each DEFCON level, there are specific actions people have to take: The real details of those actions are secret, but at DEFCON 3 -- I'm making this up -- you might call everyone back from leave, at another you fuel all the bombers, at another you arm the bombs, and so on.
Knowing the current DEFCON level is important for those in the military, because it dictates what actions you should be taking. What am I supposed to do when the terrorist threat level is yellow? Or orange? I have no idea.
And no one else did, either. Were there plane trips you delayed when the level was orange that you made when it was yellow? Did any company base business decisions on it? Do we think the president consulted the level every morning?
Consider hurricane warnings. Hurricanes are short-term events, and it's obvious when the danger is imminent and when it's over.
People can do useful things in response to a hurricane warning -- board up their windows, stay in the basement, evacuate -- and there is a discrete period when their lives are markedly different; there is utility in the higher alert mode, even if nothing comes of it.
Compare people's reactions to hurricane threats with their reactions to earthquake threats.
According to scientists, California could experience a huge earthquake sometime in the next 200 years. Even though the magnitude of the disaster will be enormous, people can't stay alert for two centuries.
