Yellow flags mean A single yellow indicates danger ahead, such as debris from a crash. Drivers must slow down as they pass; no overtaking is permitted, unless it is unavoidable such as a driver retiring in the section, or a driver is lapped.
Two waved yellows at the same post indicates great danger ahead. Drivers must slow down and be prepared to stop; no overtaking is permitted unless a driver is lapped.
Yellow flags and the SC board (a large white board with "SC" in large black lettering) indicate that the Safety car has been deployed. Drivers must slow down, not overtake and be prepared to leave the normal racing line or even stop as a threat obstructs all or part of the track.
A green flag indicates that any previous danger has been attended to. The track is now clear, and drivers may proceed at racing speed and may again overtake. When the race director so directs, this may be displayed during the parade lap or at the beginning of a practice session; in this case all marshals positions will signal green flags. Green flashing lights are used in addition in modern races.
A red flag indicates that the race, practice session, or qualifying session has been suspended. All marshal stations will signal this. Drivers may not leave the pits. All drivers on the track must proceed cautiously to the red flag line and stop. There they will be reordered in their correct racing order. Sessions may be resumed or abandoned as the race director indicates. Flashing red lights are now used in addition to the flags. If the safety car is deployed, the racing cars should follow it and provisions allow for the safety car to divert the field into the pit-lane and wait there. Other than that, drivers who enter the pits will be given a drive-through.
At any time, a stationary light blue flag (or, as is now more common, a blue light) may be shown to a driver at the pit lane exit to warn them that cars are approaching on the track.
During practice, a light blue flag waved on the track notifies a driver that a faster car is about to pass them and they must move aside.
During a race, a light blue flag waved on the track warns the driver that they are about to be lapped by a faster car and must let it pass. A driver may incur penalties if they ignore three successive blue flags.
A white flag indicates a slow-moving vehicle such as a retiring car, an ambulance or tow truck ahead on the track, and instructs drivers to slow down.
A black flag orders a particular driver to return to his pit within the next lap and report immediately to the Clerk of the Course, usually because they have been disqualified from the race. The flag is accompanied by a board with the car number of the driver on it so no mistake is made.
A black and white chequered flag signals the end of the race, practice session, or qualifying session. During the race it is shown first to the winner and then to the rest of the field as they finish; otherwise it is shown at a predetermined time.
A half black and half white flag informs a driver that their behaviour has been deemed unsporting and if they do not begin acting in a sporting manner immediately they will be disqualified. A sign with the car number accompanies the flag.
A black flag with an orange circle (40 cm in diameter) in the centre informs a driver that their car has a mechanical problem that has the potential to harm themselves or other drivers and they must return to their pit. Shown with car number.
A yellow flag with red stripes warns drivers that the track surface ahead is slippery, or there is debris present. This could be as a result of a car spilling oil (or some other engine fluid), or because rain is starting to fall. Slippery runway in an area, either by water or oil. Drivers must slow down at that point.