I didn't know that it really is good to know as while most places I would visit in Japan would be accessible via Shinkansen and a bicycle there would be times I would like to go to the countryside where driving is required and if you live in Japan that would be good to know. I have heard that the speed limits are quite slow on the highways though besides the concern of driving on a narrower street.
With a rental car you'll always have insurance, although it is worth reading the terms and conditions in order to avoid unpleasant surprises!