Phil de Conch's interesting post about different signals on different islands led me to this -https://www.boat-ed.com/pennsylvania/studyGuide/Communicating-With-Sound-Signals/101039_101039081/
And in the UK in the 1960's it was customary to put headlights onto main beam when overtaking, but it was a dangerous practice, dazzling the overtaken driver and oncoming traffic, and soon died out. Flashing headlights is now usually an "invitation to proceed" to another driver, but needs to be interpreted carefully.