The government of Somoa recently changed the side on which its cars travel on the road from the right side to the left side. Two reasons were offered for making this drastic change. One was to bring Somoa “into line with other south pacific nations” and the other was “to end its reliance on expensive, left-handed drive imports from america.”
Most roads in the world (about 72% according to Wikipedia) carry traffic on the right side of the road.
The countries that have cars on the left side of the road consist mainly of Britain and its former colonies, and Japan. Looking at the past, it made sense to travel on the left side of the road. Since most people are right handed, the knights/samurais found that it was easier to draw their swords and defend themselves against an attack much more easily. Also, it is easier for a right-handed person to mount a horse from the left side, especially if you have a sword hanging from your left side. Finally, “it is safer to mount and dismount towards the side of the road, rather than in the middle of traffic, so if one mounts on the left, then the horse should be ridden on the left side of the road.”
Historically, the reasons to travel on the left side were instrumental for Britain and Japan; however, their reasons to continue travel on that side of the road are very ceremonial.