Road Regulations by Country
Arguably, the best source of information on traffic rules for a country is that country's Ministry of Transportation. It makes sense for someone organising a road trip to contact the relevant authorities in the country they wish to drive into, to get the most up to date information available.
In principle, international road and traffic regulations were defined by the 1968 Vienna Convention on Road Traffic. Although not signed by all existing countries, and although signatories are allowed to implement local variations, the Convention does provide interesting general purpose guidelines for travellers wishing to gather information on driving abroad. Indeed we've used it as reference on TSE several times when discussing road regulations.
Road Regulations in the European Union
There's a page on the European Union website listing the road traffic regulations by EU country with downloadable PDF documents. The gathered regulations rules cover speed, alcohol, Day time Running Lights, winter tyres and mandatory safety equipments to be carried in the vehicle. There is also another page on the same website which gathers more regulations per country, including mobile phones, seat belts, traffic lights and a few others. Click on the country to see the details.
In addition, the AA website has a dedicated section with tips for driving abroad. Moreover the AA also provides a webpage with downloadable PDFs of driving tips by country. Being a British association however, the tips are mostly about driving in Europe, which somewhat makes sense.
How about the Rest of the World?
At the cost of sounding repetitive I'll go back to my previous statement:
Arguably, the best source of information on traffic rules for a country is that country's Ministry of Transportation.
Having said this, Wikipedia has a page on Road Signs by country, some of which even have their own detailed dedicated page (if you can't find them in the table of content, look at the bottom of the page for country-specific links). Should the country you are looking for not be listed, a quick google search of the kind
[country] road regulations should yield informative results from authoritative sources.
The Risks of Driving Abroad
To wrap up I'll say that driving abroad can be a risky business. Setting aside road regulations which might differ, some countries also follow unwritten rules that are often taken for granted by local drivers. Other countries might have very detailed road regulations, but those might be available only in the country's language (I'm pretty sure that for example Brazilian road regulations exist, but the easiest way to find them would be to type search keywords in Brazilian Portuguese).
In addition, the inherent risk in driving abroad increases the more one moves to emerging and developing countries, where road regulations can lag behind those of more developed countries, or are even sometimes non-existent. Should you ever venture in any such countries with the intent to operate a vehicle of any kind the best advice I can give you is to keep calm and drive defensively. If hell breaks loose around you ignore it and stick to your safe driving principles. The second best piece of advice I can give you is to ask locals how they do it, and what are the things you must know and do when driving in their country. At this point you'd expect me to finish off by saying that the third best piece of advice would be not to drive in such countries. I won't. Drive if you want to drive, but be safe.