It is a Catch-22 as far as the "terror alert" goes. If nothing happens - then you get complacent. If something happens - then you can at least say you were warned. But if you can't really do anything differently as a citizen other than, as the mayor and the rest of the politicos say - go about your business and be alert - I'm not sure that the public alert is the way to go.
As an example - how do they do this in Israel? My guess is that there is a constant high-level of security as befits a country under attack - and that the public isn't jerked around with terrorist warnings unless they are specific.
How about this criteria: you get some information that certain buildings are potential targets, though you don't have any time frame. Notify the security for those buildings. Bring in the flack jackets etc. But certain things - like closing particular bridges for trucks - that should be the current state of alert. In other words - does it take documents from Pakistan to know that truck bombs are modus operandi for terrorists? Of course not.
If, and only if, you get some information where the behavior of citizens in the city should be altered - then let us know. But I will admit - as I said - it is a close call. I'm just afraid that if you keep warning us - and nothing happens - that eventually you will warn us and we will ignore you.