Flag down

You often hear the expression “flag down a cab” – even though it simply means waving one’s hand to attract the driver’s attention so he’ll stop. Originally this phrase came from the vocabulary of automobile racing, where the officials signal to the winner that the race is over by waving a flag up and down. This signal is also sometimes used to warn other drivers they should stop or proceed with caution. Trains also are flagged down when emergency conditions warrant and timetables sometimes indicate flag stops, where through trains will stop only when flagged by the stationmaster.