Jan. 6, 2004: CBC TV's Neil Macdonald reports on the U.S.'s latest security measures in its fight against terrorism (Runs 3:34)

The U.S. colour-coded threat alert system began in March 2002, six months after the attacks on the twin towers in Lower Manhattan and the Pentagon. Its five levels, ranging from low to severe, represent the varying risk of terrorist attacks.

Britain followed suit a few years later after four suicide bombers attacked London's transit system on July 7, 2005, killing 56 people and injuring 700 others. Just after the attack's one-year anniversary, Britain approved its own five-tier alert system based on the U.S. model. The new system, ranging from low to critical, launched on Aug. 1, 2006.

On Aug. 10, 2006, the message to the public was clear. Both the U.S. and Britain flagged the highest possible security alerts — the first time for the U.S. Those alerts came after British police said they arrested more than 20 people who allegedly planned to blow up airplanes headed for the U.S. with explosives smuggled in carry-on luggage.

The U.S. government assessed the threat as red or "severe" for commercial flights from Britain to the U.S. The British government raised the threat level to "critical," meaning "an attack is expected imminently."

But what do these alerts actually mean?

How is 'threat' information evaluated? How are the levels set?

In the U.S., the attorney general and the assistant to the president for homeland security assign the threat conditions. They consider the following factors when assessing "threat information."

1. 1. To what degree is the threat information credible?
2. 2. To what degree is the threat information corroborated?
3. 3. To what degree is the threat specific and/or imminent?
4. 4. How grave are the potential consequences of the threat?

In Britain, the Security Service assesses the threat levels of domestic terrorism. The Joint Terrorism Analysis Centre (JTAC) sets international terrorism threat levels and it informs ministers of its decision. The centre has staff from the Defence Intelligence department, the Foreign and Commonwealth Office, the Home Office and the police.

What happens when a threat alert is triggered?

In the U.S., the different threat levels trigger the following responses:

In Britain, the five threat levels are divided into three response levels:

What kind of an impact have these alert systems had?

It's hard to tell how effective these alert systems are, but there have been reports that frequently changing the alert codes creates what some have called a "crying wolf syndrome."

For example, before implementing the five-tier system, Britain had a seven-level threat alert system, ranging from "negligible" to "critical." This system was in place at the time of the London bombings. Just before the attacks, the threat level was reduced from "severe general" to "substantial," which the British Parliament criticized for giving the general public inappropriate reassurance.

It has also been noted that in the U.S., even with an orange alert, police now maintain eight-hour shifts instead of the 12-hour shifts that were ordered when the first orange alerts were instituted.

Linda Feldmann of The Christian Science Monitor wrote in June 2003 that people are becoming inured to the alerts and thus have lowered their guard even when the alert hits orange. She quoted Randall Larsen, a senior fellow at the ANSER Institute for Homeland Security in northern Virginia, as saying: "If we go from yellow to orange, the threat of an attack is higher. But is that the threat of a small car bomb or a nuclear weapon? I'm going to worry about one much more than the other."

Larsen says for practical purposes, we live in a two-colour world — yellow and orange 2 and that the two lower levels — green and blue — are "politically infeasible." As for the highest alert, Feldmann quotes Larsen as saying that, in reality, "Code Red will mean an attack is underway."

Some cities are becoming weary of the shifting alerts, another aspect of the "crying wolf syndrome." Feldmann said many cities across the U.S. are complaining of the costs of police overtime when alerts rise. In Portland, Ore., for instance, extra duties cost local police an additional $365,000 in overtime in 2002 just to protect the city's bridges.

Another aspect of the heightened alerts is that they might be as much for bureaucratic reasons as because of genuine security fears. As Feldmann suggests, the alert system may be nothing more than "a means by which the government can protect itself from public criticism in the event of another attack."