Long before two-way radios were installed in cars to allow drivers and crews to talk with each other, the only way for racing officials to communicate with drivers during noisy racing action was with the use of colored signal flags.
Nearly every racing series in the United States uses the same combination of flag colors, usually waved by a flagman on a platform at the start-finish line, so drivers know immediately what's happening during the race.
Just like the green, yellow and red signal lights at a traffic intersection, some of the flags communicate racing conditions.
GREEN: The track is clear and cars may proceed at speed. This flag is used to signal the beginning of the race and any restarts.
YELLOW (CAUTION): The track is not clear, slow down and hold your position behind the pace car. This flag is used to signal an accident, debris caused by contact or mechanical failure, or weather-related issues. NASCAR rules allow cars to bunch up behind the leader. In most cases, lead-lap cars restart in the outside lane, while any lapped cars restart to the inside. In addition, a yellow flag during a practice session means cars should go to pits immediately.
RED: The track is unsafe and there is a situation that requires immediate attention. Cars must go to a designated location and stop. This flag is usually waved in cases of heavy precipitation, an accident which requires immediate medical assistance or if the track is blocked. In addition, NASCAR reserves the right to throw a red flag in the closing laps of a race to make sure the event ends under green conditions.
WHITE: There is one lap remaining in the scheduled distance.
CHECKERED: The event has reached its scheduled distance and is complete.
In addition, there are flags that communicate information.
BLACK: Come into the pits immediately for consultation. Normally, this flag is waved at an individual car, either because it has a mechanical problem or has broken a rule. Waved in combination with a red flag signals the end of a practice session.
BLACK WITH WHITE CROSS: Cars that refuse to acknowledge the black flag are shown this flag, which means NASCAR will no longer score them until they come into the pits for consultation.
BLUE WITH YELLOW STRIPE: Pay attention to your mirrors, because a faster car is approaching from behind. Contrary to popular opinion, this flag does not make it mandatory that slower cars must move over for the leader of the race. This flag gets a workout at tracks like Bristol and Martinsville, where traffic becomes a major factor.
YELLOW WITH RED VERTICAL STRIPES: Used only on road courses by corner workers, held or waved to signify debris or slippery conditions ahead.
