If someone flashes their headlights at you three times it means that they are warning you of a hazard ahead, like an accident, debris on the road, animal in the roadway, speed trap etc.
At night if someone flashes their headlights at you three times it means that you have your high beams on or that your headlights are not on at all.
The main message of someone flashing their headlights at your three times in the day is to be cautious and aware of your surroundings.
The driver also can be trying to tell you that your headlights are of and you should turn them on.
When a trucker flashes his lights at you when you pass, it means they are giving you a safety courtesy signal that means that it's "all clear to merge back,".
This indicates that you have enough room to get back into the truckers lane.
When a trucker flashes his lights at you when you pass it can also be a warning about any hazards that are ahead, such as a police speed trap, accident even an animal or object on the road.
In some cases if the trucker flashes their lights at you when you pass, it can be a sign of road rage, but road rage is less common as the reason trucker flashing his lights at you when you pass.
The most frequent use and meaning of a trucker flashing his or her lights at you, especially when passing is to say all clear to merge.
A trucker that flashes his or her lights at you once or twice is a way to let you know that you have completely passed their cab and their trailer, and that it is now safe to move back into the lane.
And if the truck driver is thankful, they may also blink their marker lights once to say thank you.
The flashing lights on a semi can also be a warning to let you know to slow down or be aware of something ahead, which could be a police car using a speed trap or accident or an obstruction in the road.
Sometimes a quick flash from a trucker is just a way to acknowledge another driver's presence.
And at night, a trucker may flash their lights to let you know you have your high beams on and are blinding them.
And if that is the case you should turn your high beam lights off until the truck has passed you.
If you're in the process of passing the semi, a quick flash from a trucker is almost always the truckers way of saying you are clear to get back over.