I was doing some research for a presentation a few months back, and I learned the interesting history of why we drive on the left or right side of the road. I ended up not including this in my final presentation, but I thought it interesting enough to share anyway. This is probably incomplete as most of the references I found came from the West, so if you have any other references, please share.
As with many things in Western history, it has roots in Rome. One of ancient Rome's greatest inventions were paved roads – all roads lead to Rome and all that. Their vast network enabled their armies to travel willy nilly, relatively speaking, across their vast empire at speeds never before seen.
Archeological evidence at a Roman mine showed deeper wear on the left side of the road. This suggested that the heavily-laden carts were coming out of the mine on the left.
Considering that the Roman Empire was once 2.51 million square miles (6.8 million square kilometers), this greatly influenced the rest of the continent's road habits.
Another thing that suggests the early European habit of riding on the left side of the road is the fact that most people were (and still are) right-handed. This meant that fighters, especially on horseback, stayed on the left side so their weapons were kept on their right.
This habit seemed to go on for a few hundred years, probably reinforced by the near-constant warfare that seemed to be the primary form of entertainment before TV was invented. And then this dude came along with his conquering armies across Europe.
Napoleon was reportedly left-handed, and whether it was due to this, he just wanted to make a point, or just because he wanted to piss everyone else off, he had his armies march on the right side of the road. So for a while, this Napoleonic French habit influenced the empire's colonies, occupations, and allies. Their continental rivals, the British, continued riding and driving on the left side of the road. I imagine this was partly due to cultural inertia and just because it wasn't French.
Originally, the British colonies across the ocean adhered to the left-side rule, but due to a deep disagreement that boiled over in the late 18th century, lots of these old British habits were set aside. Anything that was anti-British probably got a decent amount of popular support during this period in what became the United States.
These European and American habits spread across their respective spheres of influence over the next couple centuries and continued via cultural inertia, even as the primary mode of transportation evolved from horses to automobiles.
In more recent history, some countries have swapped from one side to the other. On September 3, 1967, Sweden decided to swap from left to right. Their neighboring border-sharing countries all drove on the right, so there were good reasons to change, but it apparently took 40 years of debate before the law was finally passed.
Most recently on September 7, 2009, Samoa also changed the rules of the road making the switch the other way, this time from from right to left. Their reasons for changing were primarily economic. Their closest international & economic neighbors drove on the left: Australia, New Zealand, and Japan. It was far cheaper to import cars from those countries than from the United States. As you can see below, the transition wasn't pretty.
If you have anything to add to this, I would love to learn more about it. I will try to do additional research on Eastern cultural history to see how they started, and I will follow up once I have more to share.