On the “thumbs-up” thing, I think I’ve read somewhere (but don’t remember where) that in some middle eastern countries this signifies the same thing as the raised middle finger in other places.
I use the wave while driving, to mean “Thank you.” I don’t recommend using the ASL one above, especially if you’re turning.
The only time I flash my headlights at someone is to mean “Hey! You forgot to turn your headlights on!” I don’t know if that’s universal, but it seems to me that if you saw flashing lights behind you, your mind at least might drift toward the subject of lights, and you’d notice that your headlights were off.
I do the same; someone doesn’t have their lights on when they clearly would want to, I give them a quick flick of the lights. It’s also used (around here at least) to warn of a cop ahead so that you can check your speed. Of course, cops are savvy to this, so they have pulled people over for flicking their lights. One case that I remember, a young guy had flashed oncoming traffic to warn of a cop. A cop headed in the opposite direction noticed him - and of course knew why he’d flashed. But he pulled him over anyway on the premise of thinking maybe something was wrong - maybe he was attempting to signal the cop. Of course, he wasn’t, and ended up busted for a joint in his ashtray. I’ve always been a little more wary of flashing a warning after reading that.
As to the OP, I always give a quick wave, which isn’t always easy since half the time I’m shifting into the next gear with one hand, and driving with the other. And it’s probably irrational, but it annoys me slightly if I go out of my way to let someone in front of me and they don’t acknowledge it somehow.
A quick wave with your right hand is very common around here, too.
Tapping your brakes is usually done to encourage tailgaters to back off a bit.
Flashing your headlights is used to tell people to turn their own headlights on.
Flashing oncoming traffic is also done to warn people that they’re approaching a speed trap 
Obviously not. Because the OP doesn’t know it. Therefore it’s not universal. 
I have often thought it would be good if cars had a rear-mounted courtesy light. Perhaps a purple strobe that flashes twice to say, “thanks for letting me in”. And maybe flashing four times when you mean to say “Sorry, I goofed”, like when you reverse out of a carpark without looking properly. They could become as standard as indicators and brake lights.
What parts might that be. I’d like to check it out. My wife was Italian and some of her family members had only been in the US for a generation or two. I’ve never heard any of them have a negative response to a simple wave. Now there are some more complicated hand gestures that will get you in a fight.
However, there may be some isolated pockets where a wave could be considered an insult. I guess ThatGuy better be careful when driving to those places, huh.
This is GP therefore I agree that my statement. “A simple wave is good anywhere.” is too absolute when taken literally. I usually try to avoid making absolute comments such as the previous one. Therefore I would like to amend it to: A simple wave is usually good anywhere. or How about, A simple wave is good in most places? Will that suffice?
Good, I hope so too.
In the UK, an open palm wave is used, or a quick flash of the hazard lights.
Simon
So what does this gesture mean?
Put your hand with beneath your chin, with the palm more or less facing your throat. Now bring your hand forward, gently brushing your chin with your index finger and end up with the palm facing up. Perhaps you should also have a mean look on your face. :mad:
I know - or rather think - it´s a gesture of contempt, but how deep does the feeling go? In other words, if you were to express this verbally, what words would you use?
I suspect it´s is an Italian gesture, but I never saw it used in any of the Godfather flicks.
Yah, it’s Italian.
Basically means “screw you”.
Universal? Probably not. The “OK” gesture (circle with thumb and index finger) is a hand signal for “asshole” elsewhere- I saw someone use this while driving yesterday (directed to the person in front of us, not to me ;D). The “thumbs up” is recognized as a representation of the male naughty bit… so it might be interesting if you thanked a female driver with this one. The peace sign also has some pretty racy meanings elsewhere in the world.
I have a sister-in-law that trained tour guides for Disney for a while, and supposedly they have some sort of internal handbook on potentially offensive body language from around the world. Pretty much the only things I remember taking away from that conversation are that Disney guides point at things with their index & middle fingers together and avoid making fists.
I generally just do an open palm, fingers together, pope wave kinda thing… I don’t try to make eye-contact or smile, as I’m usually busy merging into traffic or whatnot and have my attention elsewhere.
On the subject of misinterpreted gestures:
I took a large lecture course in college where one of the students was deaf, so a translator stood next to the professor and repeated everything she said in sign. The professor had a triangle shaped hand gesture that she always used when talking about . . . something, I don’t remember. But anway, it must have been some point she made a lot, because she did this almost every class. One day after class the translator commented to her “You know, you keep giving the sign for ‘pussy.’”
I would advise using this in traffic, as it is far too easy to be misconstrued by an ignorant person as an insult.
Maybe this one: Holding the right hand, with the back forward, then lightly brushing the tips of fingers beneath the chin several times with a forward motion has the meaning of an insult.. (Page 62 of cached document in google link. The pdf original did strange things to IE6.)
Or perhaps one of these.