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Blue flag
This flag encourages a driver to move aside to allow faster traffic to pass.

Faster car approaching—during races
lapped cars should give way to faster cars

A light blue flag, sometimes with a diagonal yellow, orange, or red stripe, informs a driver that a faster car is approaching and that the driver should move aside to allow one or more faster cars to pass. During a race, this would usually only be shown to a driver getting lapped, but during practice or qualifying it could be shown to any driver. In most series, the blue flag is not mandatory—drivers obey it only as a courtesy to their fellow racers. As such, it is often referred to as the "courtesy flag." In other series, drivers get severely penalized for not yielding or for interfering with the leaders, including getting sent to the pits for the rest of the race. In Formula One, if the driver about to be lapped ignores three waved blue flags in a row, he is required to make a drive-through penalty. The blue flag may also be used to warn a driver that another car on the same lap is going to attempt to overtake them.


Taken from Wikipedia

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