Hey, I'm an American living in northern Japan. I have read some interesting posts here and hope sometime I'll have something to add. I have been cycling for about three decades now, but only seriously on and off. Nowadays I do a little off road riding, some fire/logging roads, and some road riding. I have a front suspension Louis Garneau mountain bike, but a few years back realized that most of my riding didn't require heavy off road tread, so I'm using hybrid inverted tread tires. I have rear panniers that I use when touring.
Up north here on the island of Hokkaido the roads tend to be wider and the cities smaller. I'm not sure if courteous is the term I would use, but people are generally careful to give you the room you need. In Japan, it is understood that in a motor vehicle vs. HPV or pedestrian accident, the motorist is considered in the wrong unless proven otherwise. So things are pretty good for cycling, especially here in Hokkaido, although I haven't ridden down south.
Quite a few cyclists come up to Hokkaido to tour in the summer, because it's generally cooler, less rainy and less humid than down south. I think the width of the roads has something to do with it to. Motorcyclists too, called 'riders' here, come in droves in the summer. And it's common for touring cyclists and touring motorcyclists to wave when they pass each other. It's interesting that when I go for a short ride with no panniers, almost no one waves, not even other cyclists, but when I go out with panniers I join the brotherhood of two wheels and pavement.