A curious thing I noticed on the way down to Canberra on the weekend - The semi-trailers that drive down the highways seem to have a special set of signals they use, when over taking each other.
Obviously because of their size the drivers don't have good visibility around them, so when one semi-trailer overtakes another, the 'overtakee' will flash their headlights to tell the overtaker that they are clear to move back into the lane without hitting anything.
The overtaking semi indicates left*, and moves over to the lane, then flicks the indicator over to the right for one flash, then back left for two flashes.
I'm guessing it's a 'thanks mate' signal, but I also noticed that if the overtaking semi-trailer plans to overtake another, or at least stay in the right lane, after the over-taken truck flashes their headlights, the overtaking truck will indicate to the right for two flashes, then left, then right again, as if to say 'thanks, but no thanks'.
Its interesting to think that every time we saw one semi-trailer overtaking another, it happened. So it seems to be a communication that is common between drivers, at least in that region. I guess it can be compared to the two-finger wave (courteous) drivers give each other when we let each other in, changing lanes etc.
Since we were driving at night, we noticed the huge flash of light from the truck's headlights as they overtook each other somewhere behind us. It's kinda spooky if you don't know whats going on.
*Australian roads