I hope people don't just cross Japan off their list. It is still a lovely country to visit. The reason I am not going to visit again is my lawyer told me If i visit Japan I will be questioned by an Immigration Officer. So that means a chance I wouldn't be let in. I am not going to risk that.
I would be wary of hiring a rental car. Most rentals will bbe no problem at all but accidents do happen. If someone has any injuries you could end up in my position in Detention. The real problem are the motor cycles. Some as in all countries are a problem and certainly had other instances where the overtaking has been dangerous. But the other problem with them is if in an accident they will very likely come off their bike and be injured. It is the other driver that then has a problem as they caused an injury.
Now first if you do rent a car in Japan take out their top insurance. It really does work. I didn't have to pay out a yen and the other fellows injuries were covered.
If you are in an accident every piece of advice I have seen is to insist on your right to be silent until you have seen your lawyer. I now know why. Sorry but the police basically stitched me up by A. Not investigating the other vehicle and probably B. by lying about the existence of a camera at the previous intersection. The lawyer doubted it's existence. But if you think you are on film then why not admit what you did.
Second remember that as soon as they put you into detention you will not be able to contact anyone and they take all your property from you. So give your phone etc to whoever you are with and tell them to contact any one who needs to know. Also a great idea if they have a list of English speaking lawyers or know how to find such a list. There is one on the Embassy in Tokyo's website.
If by yourself I would early on email a friend telling them where you are and what might happen and if they don't hear from you by the next day to let the Embassy know.
Also have a plan as how you are going to get Japanese cash-yen only accepted for payment of fines and no sort of card accepted.
Then you just have to put up with the routine and boredom. try and keep your sense of humour. That helped me a lot. When interrogated I usually had this song going through my head.
The guards also helped though they were unaware and thought it odd that I would break out into a smile when a meal was delivered. With the meal you got a rolled up plastic place mat with one of 2 messages on it in Enlish - would you like to feel better or would you rather be somehere nicer.
Also look for anything nice in those around you. As I have said I was treated well by one young guard, the female Paddy Wagon driver and the local interpreter. You don't want to lose your own humanity.
Over the next few days I will post the advice I received from the first visit by the local lawyer I never got to see and that from the Embassy. Also some other sources I have found online.