What does it mean when a tractor-trailer flashes his brights on a US highway?

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Flashing lights has several meanings:

  • It is a warning. In many countries if the oncoming traffic does it, it means "Hey, check your car" which is nearly to 99% that you forgot to turn on the lights or forgot to turn off full beam (you are blinding them). If your car is ok, it means most likely that danger is ahead (accident, unofficially speed control). If the car is behind you, it can also mean that something is wrong with your car or it means "Hey, be careful" (it looks like you are drunk or you had microsleep). If the car is faster than you and wants to overtake, flashing warns you of its presence.

  • It is displeasure. If someone is directly behind you on the overtaking lane, flashing means Hey, give way. It is also mixed with warning if the person doing the flashing thinks you are driving bad, e.g. driving too fast/too slow.

  • It indicates help. If e.g. I see that a car/truck wants to overtake before me, I adjust speed and give a short signal to indicate: I have seen you, you can changes lanes. If two cars passes in a constriction, the car who is willing to wait flashes.

This are the most common and nearly universal indicators. If there is reason to thank the person behind you (you are overtaking and the car on the lane gave you way) or acknowledge (you really had microsleep), it is polite to touch gently and shortly the brake two times, so your follower sees two short red flashes indicating "thanks".

Upvote:10

This Answer requires some presumption based on experience. But...

  • The driver is alerting you to the presence of a large vehicle behind which in low visibility situation, you might not see. That vehicle requires substantially more stopping distance than a car.
  • The driver saw you swerve, thought you are a bad driver and is expressing their displeasure.

While not this case, truckers flashing lights to oncoming traffic has been a signal that law enforcement is stationed ahead.

As for your previous experience, while you might not think you cut them off, professional drivers have a different, less tolerant, view of such things.

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