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I live in northern Japan and people drive very slow. The roads are in bad condition with the exception of a few good ones. It's funny sitting at a red light. Some Japanese people will turn their car off. Not sure about rules with car seats. I see kids all the time standing up and jumping around in the cars while they are traveling. Also once you get on the express ways there are toll Booths pretty often.

Drivers are very polite. In general people drive very slow, and the stop lights are very long. There's lots of pedestrians and they have the right of way. On the expressway things change, you can pretty much drive as fast as you want. The roads are generally in good condition. They don't plow their roads much and don't seem to use salt, so be prepared to drive carefully if snow is expected. Traffic is usually less than you might expect, but if a traffic jam does occur, it's usually pretty epic and you'll be sitting around for hours. Parking is abundant if you know where to look and are willing to pay. My experience is a car with 3 people is cheaper than train tickets. But it may not be worth the effort for driving around the city. I just park it somewhere and walk. The car is certainly convenient though. My best advice is to consider your car as an additional member of your group, which means you have to plan around it. You'll be fine if you make plans for your car.

I agree driving on the tollways seems to permit any speed, even though there are usually 80 kph limits.
In general, the Japanese seem to be quite polite, courteous  etc.  I've been in Sapporo 2 years.
It's been a pleasure to be here.