Red flags can signify a warning, martial law, defiance, or left-wing politics. The earliest citation for "red flag" in the Oxford English Dictionary is from 1602 and shows that at that time the flag was used by military forces to indicate that they were preparing for battle. It has been associated with left-wing politics since the French Revolution. The red flag became a symbol of communism as a result of its use by the Paris Commune of 1871. The flags of several communist states, including China, Vietnam, and the former Soviet Union, have red backgrounds. The Labour Party in Britain used it until the 1980s and the French Socialist Party uses it. The earliest citation of "red flag" in the sense of a warning is dated 1777 and refers to a flag warning of flood.
As early as the 15th century, the red flag was used as a "flag of defiance. It was raised in cities and castles under siege to indicate that they would not surrender. "The red flag is a signal of defiance and battle," according to Chambers Cyclopedia (1727–41).
The color red become associated with patriotism early in the French Revolution due to the popularity of the Phrygian cap and designs based on the Tricolor Flag, both introduced in 1790. A red flag was raised over the Champ-de-Mars in Paris on July 17, 1791 by Mayor Jean-Sylvain Bailly as a symbol of martial law, warning rioters to disperse. Over fifty anti-royalist protesters were killed in the fighting that followed. Oddly inverting the original symbolism, the Jacobins protested this action by flying a red flag to honor the "martyrs' blood" of those who had been killed. The Jacobin Club ruled France during the Reign of Terror (1793-94) and made the Red Flag an unofficial national flag. However, the earlier Tricolor flag never lost its official status and regained popularity under Napoleon.
British sailors mutinied near the mouth of the River Thames in 1797 and hoisted a red flag on several ships. The flag was also used during the Merthyr riots of 1831 in South Wales. During the 1848 Revolution, Socialists and radical republicans demanded that the red flag be adopted as France's national flag. Led by poet-politician Alphonse de Lamartine, the government rejected the mob's demand: "[T]he red flag that you have brought back here has done nothing but being trailed around the Champ-de-Mars in the people's blood in 91 and 93, whereas the Tricolore flag went round the world along with the name, the glory and the liberty of the homeland!
The banner of the Paris Commune of 1871 was red and it was at this time that the red flag became a symbol of communism. The flag was flown at a rally in Chicago on May Day 1886, which resulted in the execution of some of the Haymarket Eight. The red flag gained great popularity during the Russian Revolution of 1917. The Soviet flag, with a hammer, a sickle and a star on a red background, was adopted in 1923. Various Communist and socialist newspapers have used the name The Red Flag. The red flag, and the color red generally, was adopted by the Communist Party in China, where red is associated with good luck.
In more recent times, social democratic parties have gravitated away from the Red Flag as a symbol. However, several European parties retain a "red square" symbol, including Germany's SPD and the Party of European Socialists.
This flag is typically used after a major accident where safety marshals are required out on the track for longer than desirable period, and having cars drive past, even slowly would pose a significant danger.
This flag can also be used where weather conditions and or the track surface prevent racing from been undertaken safely.
It is usually up to the race organizers as to how they wish to handle the race after that. They may either decide to cancel the race and declare no winner, cancel the race but declare a winner and award race points, or they may chose to restart the race at a later time or date. If a restart happens then it is up to the organizers to decide if the race will restart with the same number of laps or with a reduced number of laps.