I've been driving in Japan for over 15 years now. One of the things that always resonates is how polite people are on the road, especially the way drivers say thanks with their hazard lights.
[GIF via Prince Kevin]
I don't think this practice is unique to Japan. It probably isn't. While living in the US, however, I never did it and don't recall seeing others do it. But being able to thank others and be thanked, certainly makes the roads a more cordial place.
Japanese society is often, but not always, quite mannered, so it would make sense that the motorways are, too. For example, cars often heed the right of way to faster vehicles as not to get into their way.
But the most common way drivers in Japan communicate is through what's called the "thank you hazard" (サンキューハザード or sankyuu hazaado).
It's used after another driver lefts you in or lets you pass as an expression of gratitude. It can also be a way to apologise after you suddenly want to change lanes, and another driver, maybe begrudgingly, lets you cut in.
If your lane is blocked and you need to get over briefly and then return to the lane, you'd probably say a quick thanks with your hazard lights.
Traffic News (via NicoNico News) recently interviewed a driving instructor surnamed Tanaka from Fuji Driving School in Tokyo, and according to the instructor, the "thank you hazard" isn't taught to students. Everybody does it on the road, so people adapt the practice naturally.
But how did the practice begin in Japan? According to Tanaka, drivers in cars would open their windows wave to express gratitude. That's more difficult for truckers, who would flash their hazard lights instead, and that's how Tanaka thinks the practice spread in Japan.