A blue flag tells a driver that a car about to lap them is right behind, and requires them to move aside at the first safe opportunity. In practice that means roughly three marshal posts, about half a mile. Ignore three consecutive blue flags and a five-second penalty typically follows.
It is the only rule in Formula 1 that asks a competitor to actively make their own race worse, and that is precisely why it produces an argument every few weekends.
When is a blue flag shown?
In a race, marshal posts show a blue flag to a driver who is about to be lapped, once the car behind gets to within roughly a second.
The obligation is to let the faster car past at the first available opportunity, which is the wording the FIA's International Sporting Code uses. Not immediately, not dangerously, but at the first place where it can be done safely.
Practically, drivers get around three marshal posts to comply.
