In Formula One motor racing what does a yellow flag indicate?

All forms of motorsport use a system of flags or signal lights to inform drivers of hazards during a session. While technological advances has meant that, in Formula One, many flags have been replaced by signal lights, the system of coloured flags has remained constant since the early years of racing.

Single stationary yellow flag means overtaking prohibited due to danger near the track. Single waved yellow flag means overtaking prohibited due to danger on the track. Double waved yellow flag means overtaking prohibited due to blocked track. At both flags, a speed reduction is required as well.