Flags

Green Flag

In a race, the green flag is waved to signal the start of the race, and the restart of racing after a full course caution.

When cars are waiting in pit lane, a green flag may also be displayed at pit exit to indicate when it is time for cars to go on track.

In a practice session, the green flag may be waved at start/finish after a warm-up / out lap (possibly under full course yellow with passing prohibited) to indicate that passing is now allowed.

Yellow Flag

A single yellow flag warns you of an incident ahead.

As with most flags, the first thing you should do when you see a yellow flag is to check your mirrors in order to be aware of what is behind and around you. If there is a car immediately behind you, it's a good idea to raise your arm as a signal that you are slowing abnormally.

Reduce your speed to ensure that you can avoid the incident, and not lose control and become part of the incident.

Passing is prohibited until you are beyond the incident. (Presumably, if the next flag station is not displaying a flag, it means the incident was resolved before you reached it and track is clear from there.)

There are multiple uses of the yellow flag:

A waving yellow flag indicates an incident immediately ahead. A more vigorously waved flag may indicate a nearer or more serious condition. In any case, you should reduce speed and be prepared to deviate from the racing line, possibly going off the racing surface, in order to avoid a situation such as a car stopped or spun on track or near the track.
A standing yellow flag also indicates an incident ahead, but usually means that the incident is further ahead and you will likely see a waving yellow at the following station. However, this varies by track, and how close the stations are to each other.

Double Yellow Flag

A standing double yellow flag (really two yellow flags held steady togther) is a "full course caution". This means the entire course is under a yellow flag condition. Generally, all stations will display the double yellow when it is in effect. You should drive as if every station is displaying the double yellow. It is likely that a safety vehicle is also on track, and you may see a white flag accordingly.

When there is a full course caution in a practice session, you should reduce speed significantly out of an abundance of caution. But use good judgment: if you are nowhere near the incident, it is not helpful to slow to a crawl and create a traffic jam, so maintain a reasonable and safe speed.

When there is a full course caution during a race, different drivers have different obligations. Certainly, all drivers must reduce speed, especially so when approaching the location of any incidents. The race leader should reduce to pace car speed, possibly falling in behind the pace car if there is one, and allowing following cars to catch up, depending on the duration of the caution. Following cars should also reduce speed, but as long as it is safe to do so, each one should try to close the gap to the car ahead. If the caution lasts long enough, this can allow some or all of the cars to re-form as a single-file group for a rolling restart when the green flag is waved at start/finish.

White Flag

In some race sessions, a white flag at start/finish may be used to indicate the final lap. You will see the checkered flag the next time around.

A white flag is also commonly used to indicate that a safety vehicle (such as a tow truck) is on course. The white flag may be displayed at start/finish for this, and it may also be displayed along with a waving yellow at stations where the safety vehicle is currently located as it makes it way to the vehicle needing assistance. If you come upon the safety vehicle while it is moving, reduce speed and do not pass the safety vehicle until it points you by; and when passing give plenty of room. If you come upon the safety vehicle when it is providing assistance, reduce speed and drive with extreme caution for the protection of the safety workers.

Black Flag

A single black flag may be directed at a specific car. If a furled (wrapped) black flag is pointed/waved at you, it is an indication that you have been observed doing something wrong; it is a warning. You may continue, but correct your behavior.

If a normal (open) black flag is waved at you, you are being directed to go immediately to the Black Flag station in the hot pits to speak to an official. (At some tracks and some stations, the flagger may have a number board to indicate the car number to which the flag is being directed.) Proceed to the Black Flag station in the hot pits. After the official speaks to you, you will usually be permitted to re-enter the track if there is time remaining in the session. However, if you choose to enter a lengthy two-way conversation with the official, this may be delayed or you may directed to park your car.

If all stations display a stationary or waving black flag, all cars are directed to the hot pits. This may be referred to as a full course black flag, or an all-black. Proceed safely to the hot pits and line up at pit exit; wait for a signal to re-enter the track. Typically this is due to an incident on track that requires removing the cars in order to get a tow vehicle on track safely and to remove a disabled car. This usually follows a yellow flag condition somewhere on the track, but note that you may not have seen the yellow flag due to where the incident is. If the incident can be cleared before the session is over, you may be directed to re-enter the track.

Black Flag with Orange Dot

The "mechanical black flag" or "meatball flag" indicates that corner workers or staff have observed something wrong with your car. It might be dropping fluids. It might be smoking. It might have a bumper hanging and dragging or some other body panel dislodged. It might have a tire rubbing the fender. The hood may appear to be improperly latched. You may have violated the sound limit.

Check your mirrors (look for smoke behind you), check your gauges, listen to and feel your car.

If it is safe to do so, proceed to the hot pits so that your car can be checked.

If you are dropping fluids, stay off the racing line and consider stopping the car in a safe location and await assistance. Do not stop on dry grass, as that can very easily start a fire.

Debris / Surface Flag

Indicates that a substance or object is on track ahead, affecting grip or possibly requiring avoidance.

Usually the flag will be taken down after a lap or two. This does not necessarily mean the substance is gone, but you have had time to be aware of it.

When dirt or fluid is on track, this flag warns you about it, understanding that the substance may disperse as the laps go on.

Blue Flag with Yellow Stripe (Passing Flag)

When waved as you approach a station, indicates that a faster car may be approaching you from behind. Check your mirrors. When multiple cars are in proximity, the reality is that it may be unclear which car is receiving the blue flag and which car is the doing the passing.

In a race it presumably means a lead car is lapping you. The blue flag is not intended to indicate that you should yield to a car that is not lapping you.

Prepare to be passed, but be predictable; it is usually best to drive your normal line, although in some cases it may be advantageous to both cars if you "assist" the pass by easing off the throttle or providing more space. In sessions where "point-by passing" is not in effect, giving a point-by signal can still be a good thing to do, to communicate intentions clearly. Even during a race, racers will often give a point-by to signal that they know they are being passed, or when they are giving space for a pass.

Red Flag

Indicates a serious incident on track that requires all cars to stop, typically so that a safety or emergency vehicle can go to the incident directly. The track may even be blocked due to an incident.

Check your mirrors, raise your arm, slow your car safely, and come to a stop in a safe location as soon as possible. Realize that other cars may not notice the red flag at the same time as you. Stopping in a safe location is defined as:

A location off the racing line and not in a blind area; so that you will not be hit by a car that failed to slow down.
In view of a corner worker; so that you can see a signal when it is time to proceed.
Not so suddenly that cars behind you who may not have seen the flag will hit you.

Once stopped, wait for a signal from a flagger to proceed. Usually you will proceed to the hot pits or paddock under full-course black flag, but in theory it's possible to receive yellow flags which, if subsequently removed, would indicate that you may resume the session.

(In some racing, such as Formula 1, a red flag acts more like a full-course black flag as described above: cars return to the pits, without stopping.)

Checkered Flag

Indicates the end of the session. Finish the cool-down lap and return to the paddock unless directed otherwise. (After a race you might be directed to the weight scales or impound or the Black Flag station.) Unless indicated otherwise, the cool-down lap is under green flag conditions; be aware of other cars including those that may want to pass.

In a race, this is waved at the finish line when the winner approaches the line. It continues to wave for the rest of the cars.

At some tracks and in some practice sessions, the checkered flag station may not be the start/finish line, or there may be an additional location for it. This will be discussed in the drivers meeting. This is usually done in order to speed up the transition between sessions by having only a partial cool-down lap at the end.

In a race, if you are about to be lapped by the race winner at start/finish, you may see the checkered flag being waved when it is being directed at the winner who is actually behind you on track. This can cause confusion, because if it's close but you beat the leader to the line, you still have one lap to go, as the last car on track. Ideally, the flagger at start/finish will attempt to communicate the situation clearly by pointing, or even directing a white flag at you (one lap to go) along with the checkered flag for the leader.

Summary

Green Flag: Go.
Waving Yellow: Check your mirrors, reduce speed, incident immediately ahead. No passing.
Standing Yellow: Check your mirrors, prepare to reduce speed, incident ahead. No passing.
Double Yellow: Check your mirrors, Full-Course Caution, reduce speed, incident somewhere. No passing. In a race, gather the field.
Double Yellow with White: Full-Course Caution with safety vehicle ahead.
White: Safety vehicle on course, or 1 lap to go in a race.
Furled Black: Your behavior needs to be adjusted.
Waving Black: The steward would like a word with you. Proceed directly to black flag station in the hot pits.
Black Flag All Stations: All cars proceed to hot pits and wait for signal that the session has resumed or ended.
Black Flag with Orange Dot: There is something wrong with your car that needs to be addressed. Proceed directly to black flag station in the hot pits unless you need to stop the car.
Debris / Surface Flag: Something is on the track surface and it may be slippery.
Passing Flag: Check your mirrors, a faster car is approaching you. In a race, a car is about to lap you.
Red Flag: Serious incident; check your mirrors, bring your car to a controlled stop in a safe location.
Checkered Flag: The session or race is over.