The right side of the road?

This is one of those things that doesn't have a hard and fast answer, due to the fact that we cannot hop in our time machine and go back to find out.

It seems that most countries did not create any hard and fast rules for which side of a road to travel on, until wheeled vehicles were more common than pedestrian traffic.

"Keep to the left" seems to have been the custom of the Romans, and thus all the countries which they conquered or influenced. It is also believed that in the Middle Ages, you never knew who you would meet on the road. By keeping to the left, you kept your sword hand nearest passing strangers (who would pass on the right).

This custom prevailed, even in America (we undoubtedly brought it along from England at that time) until the 1700s.

In the late 1700s the U.S. began hauling products in large wagons, drawn by teams of several pairs of horses. These wagons had no seats. The driver steered by sitting on the left hand rearmost horse, which left his right hand free for the whip. Since the driver sat on the left, it is natural to prefer people to pass on the left so that the driver can be sure that his vehicle's wheels cleared yours. Thus these big wagons would keep to the right of the road.

As it happened, France was also a keep to the right country - for different reasons. When Napoleon was in power, he enforced "keep right" in all of the countries his armies occupied. Those countries who resisted Napoleon stuck to "keep to the left" as a visible act of defiance, and later a matter of pride.

Now, about Japan. Japan did not have a rule or law about what side of the road to keep to until the 1800s - until them most of the traffic was horseback or on foot and apparently they didn't feel they needed a rule. In the 1850's the British forced Japan to open their ports for trade. The British ambassador shortly thereafter convinced the Japanese to adopt the "keep left" rule.

There is a very detailed chronology including many of the countries of the world and their "keep left/keep right" rules of the road here, with detailed references.

Here is a world map showing the "rules of the road" - blue countries keep to the left, red countries to the right