In this March 12, 2002, file photo, the colour-coded terrorism warning system is shown in Washington. (Joe Marquette/Associated Press)

The U.S. Homeland Security Department is proposing to discontinue the colour-coded terrorism alert system that became a symbol of the country's jitters after the Sept. 11, 2001, hijackings and the butt of late-night talk show jokes.

The eight-year-old system, with its rainbow of five colours — from green, signifying a low threat, to red, meaning severe — became a fixture in airports, in government buildings and on newscasts.

Over the past four years, millions of travellers have begun and ended their trips to the sound of airport recordings warning that the threat level is orange.

The system's demise would not be the end of terrorism alerts; instead, the alerts would become more descriptive and not as colourful.

In the past two years, officials in the administration of U.S. President Barack Obama have changed security protocols without changing the colour of the threat, such as introducing new airport security measures after a man tried to bring down a Detroit-bound jetliner last Christmas.

By scrapping the colours, Obama would abandon a system that critics long have said was too vague to be useful and that Democrats criticized as a political scare tactic.

And it would represent a formal undoing of one of former president George W. Bush's most visible legacies.