Why Truckers Flash Their lights at each other

When a Trucker Passes another Trucker quite often you will notice that the truck being passed will turn off his lights briefly after the passing truck has cleared his Tractor sufficiently to move over. This ” flash in” also another trucker slang word, is an act of courtesy and at night or in fog can be a help to the passing truck to know that he has cleared the other drivers vehicle. Afterwords as a way of saying thank you for the flash in, the truck that has just passed will flick the interupt switch on his dash this will cause his running lights to flick off then back on, some drivers hit their High Beams instead of turning off their headlights this is sometimes o.k such as during the day time, but at night usually not appreciated very much as it can blind the driver looking into his rear view mirror to make sure he is clear to come back over.