Racing Flag White
In Formula One, the white flag is waved on the last corner and the pit straight at the end of free practice sessions on Friday and Saturday, indicating to drivers that there are drivers doing practice starts on the pit straight. Drivers are permitted to do one practice start at the end of each free practice session.
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In all championships which use the FIA International Sporting Code, as well as North American road racing, the white flag indicates the presence of an official car or a competitor moving at below normal speed in the section of track covered by the flag station.
In NASCAR, IndyCar, North American motorcycle road racing, and most American forms of motorsports, a waved white flag displayed at the starter's stand indicates the start of the final lap of the race.
In MotoGP, a white flag is used to warn riders of rain.
In some series, a white flag is shown from all flag stations on the first lap of a practice or qualifying session so competitors will know which stations are manned. Wikipedia