What does it mean when someone 'flashes' their headlights while driving?
First, flashing headlights is done one of two ways. If a driver's headlights are off, a driver can turn the lights on then back off again. If the driver's headlights are on, the driver can turn on the high beams (brights) then turn them back off again.
Part 2 is important here.
Part 2 starts with one flash which means that everything is clear. For tractor-trailors passing one another, it means that the tractor-trailer passing is clear to switch lanes. But one flash indicates ' all clear'.
Part 3 is two flashes which means trouble ahead. The trouble could be an accident, police car, or unsafe to change lanes. Two flashes means danger ahead.
Part 4 is three or more flashes which indicates that there is a total whacked out nutcase (crazy person) behind the wheel. If someone flickers there lights off and on again or multiple high-low beam changes, you need to avoid these drivers if possible. They are nuts (crazy) so slow down, speed up, or pull over in a parking lot for a bit until the nutcase passes or is out of the area. You will be glad you did.