Red flag (idiom)



A red flag could either be a literal red flag used for signaling or, as a metaphor, a sign of some particular problem requiring attention.

Background
The term and the expression "to raise the red flag" come from various usages of real flags throughout history. A red flag is frequently flown by armed forces to warn the public of live fire exercises in progress, and is sometimes flown by ships carrying munitions (in this context it is actually the flag for the letter B in the international maritime signal flag alphabet, a red swallow-tailed flag). Red flags have also been used in the medical field during position interviews because of the importance of finding the right candidates for the positions. They have also been used to identify fraud in many cases.

In many countries a red flag is flown to signify that an outdoor shooting range is in use. The United States Air Force refers to its largest annual exercise as Red Flag operation. Red flags are used for various signals in team sailing races (see Racing Rules of Sailing). A red flag warning is a signal of high wildfire danger, and a red flag on the beach warns of dangerous water conditions (double red flags indicate beach closure). Red flags of various designs indicate dangerous wind and wave conditions for mariners. In auto racing, a red flag indicates that a race has been stopped.

A study that was done on college students examines the pervasive issue of sexual and dating violence on college campuses and evaluates the effectiveness of the Red Flag Campaign (RFC), a widely implemented prevention program. The research found that exposure to the campaign increased students' confidence in intervening as bystanders. The results suggest that the RFC is effective in promoting prosocial bystander behavior, with recommendations provided for its implementation on college campuses.

In 1861, a new law in Britain set speed limits for traction engines and allowed people to request daytime bans if the engines were dangerous or annoying. Starting in 1862, these bans made it harder to use agricultural engines during the day. By 1865, another law reduced speed limits even more and required someone to walk 60 yards ahead with a red flag to warn people and stop the engine if horses got scared. This was seen as either a necessary safety measure or an obstacle to progress.

A signal of danger or a problem can be referred to as a red flag, a usage that originated in the 18th century. An infamous example of use of a red flag in warfare is Mexican General Antonio López de Santa Anna's use of the symbol to let his Texian opposition in the Alamo know that he intended to spare none of the defending slavers (on which he followed through, though the slaves were spared and freed). The term "red flag" is used, e.g., during screening of communications, and refers to specific words or phrases encountered that might indicate relevance to the case. For example, email spam filters make use of such "red flags". A red flag can also be used to indicate a small problem that could lead to larger problems in the future.