Homeland Security Advisory System: Possible Issues for Congressional Oversight [Updated December 28, 2006] [open pdf - 141KB]
From the Summary: "While the need for terrorist threat warnings seems to be widely acknowledged, there are numerous issues associated with HSAS and its effects on states, localities, the public, and the private sector. These issues include the following:  vagueness of warnings;  lack of specific protective measures for state and local governments, the public, and the private sector;  dissemination of warnings to states, localities, the public, and the private sector;  coordination of HSAS with other federal warning systems, and;  cost of threat level changes. This report will be updated as congressional or executive actions warrant."
CRS Report for Congress, RL32023