According to the Japan National Tourism Organisation, traffic congestion and parking can also be problems in cities. Of course Japan is one of the few countries which agrees with the UK about which side of the road to drive on, so that's a plus. 

But generally, hiring a car is not recommendable. People who think nothing about hiring cars all over Europe etc. should think twice about considering it in Japan, because of the writing issue. It can be very interesting to see how certain Japanese "Kanji" characters crop up again and again in signs for place-names (the characters for island, river, temple, field etc.) but the difficulty of deciphering words make it tricky to make a quick decision about which turn-off to take at the next junction! 

If you are determined to drive, you'll need to have both your normal (national) driving licence AND an International Driving Permit. People who have rented cars in Japan recommend that you get one with a satellite navigation system fitted (called Car-Navi or -Nabi in Japan), saying that even if you can't get it to talk English, it's extremely helpful.

This isn't due to any lack of literacy in the country; literacy rates are extremely high. It is down to the way that Kanji ("Chinese characters") work when used in place names. Your average kanji will have 4 or 5 different "standard" readings - sounds that are associated with it when it is used in compounds to make long words. Some kanji have rather more - 16 or 20. The Japanese learn them all at school as "standard" readings. But as soon as a kanji is used in a place name - a kanji can have pretty much any "reading" the locals care to give it. The exact same place name in one part of the country may be read in a completely different and totally non-standard way in another part. 

What's it like?
Well - imagine trying to navigate on a map in which a word spelt "B-l-a-c-k-p-o-o-l" was pronounced "Blackpool" when it referred to the northern seaside town, but pronounced "Littlehampton" when it referred to somewhere else.
And that it what it is like for the Japanese - everyone knows the readings of the major place names down to the "prefectural" level - and their own localities - but as soon as you start getting "rural", people from one part of the country don't know how to read the names from another part. They are only marginally less lost than you would be.... 

So how does this affect you - a wannabe driver?
Well - it is quite possible to drive by "squiggle". You don't have to know how to PRONOUNCE "squiggle" - you DO have to recognise it on a map. Larger place names will have English with them, smaller place names are equally baffling to everyone but the immediate locals... 

I have come to the conclusion that people can either read maps, or they can't. Those that can, can - and cope reasonably well with maps in a language they can't pronounce. They just have to concentrate harder and plan a bit more. Those that can't - well - can't... in any language, including their own. 

So the first thing I do when driving with a Japanese friend is retrieve the (Japanese) road map from her. I can't read kanji worth a damn - but I CAN read maps. So we get there far faster, and in better temper, if I navigate.
So - don't be too put off by the "road signs aren't in English" issue. It can be a bit daunting when you pick up a Japanese roadmap for the first time - but gracious me - where is your fighting spirit? 

Plan your trip the previous day with a friend, note what kanji "squiggles" you have to look out for and you're away.... If a kanji name is important to you, simply draw it on a piece of paper a few times: it will help you recognise it later.

I purchased a book of Japanese maps from maps2anywhere.com prior to our departure to help with our adventure, and off we went! The sat-nav was invaluable, even if it was in japanese ( the only option it seems ), and using telephone nos for yor destination proved an invaluable tip. We got the hire co to program the first trip and off we blundered! 

First stop was the wrong booth on the expressway, we couldn't understand what was required, but we smiled nicely and managed to make the correct payment and off we went. We arrived at our destination without too much hassle, and found the next couple of weeks of driving to be relatively stress free. We covered about 1800 kms, with two main bases, the second being up in the Japanese Alps with snow and fine views. Yes the side roads were narrow ( virtually one cars width ), the main roads were a bit stop/go, and the expressways quite expensive but hassle free, but overall quite manageable.
Stick to a small car if possible as everybody else out in the countryside seemed to. 

Cities are best avoided if possible in the car, as the trains are a better option, although we had more trouble understanding the timetables and getting on or off the correct trains than we ever did in the car! The locals were always willing to help with programing the sat-nav if required, even though we couldn't speak the language, but it was always fun trying to make ourselves understood, or trying to program it ourselves! 

So my advice would be to get out of the cities and out into the countryside to see the real Japan, where not much English is spoken and just have a load of fun (if your up for it!). Our trip , by the way, was Mar/Apr, which seemed an ideal time to go. 

Good Luck!