Race tracks have long used flags as a way for Marshals, dressed in their distinctive orange overalls, have a number of roles but the majority are positioned around the circuit... There's more... to communicate with drivers. The colour and meaning of the flags is a universal language of motorsport so there can be no confusion. If a flag is waved the OK, so we all know what the driver is, right. I've always liked the description that F1 drivers are like... There's more... must respond to it immediately. Marshals are stationed around the course such that a driver must be able to see a flag at any time.
Today a number of flags are also represented by bright LED lights so there can be no mistake.