The reason that people travelled on the left side of the road is due to horses. To mount a horse you get on the left side of the animal, for some reason they dislike being mounted from the right. Mounting a horse in the road or from the side of the road meant having the horse on the left of the road and mounting from the outside of the road, his dates back to the Romans and was spread everywhere they conquered. The origin of some countries travelling on the right and some on the left comes from Napoleon, he detested the English so much that he changed the side of the road that people travelled on in France and then all the countries in Europe that he conquered, just to be different from England. The USA followed suit after the War of Independence as a way of sticking it up the British, as well as honouring the French who were supportive of the Americans n the war. A lot of countries followed what their neighbours did as it was easier for travel and trade to be the same. The distribution around the world is due to colonization, if you were a British colony then you drive on the left, except Canada, they had French and British colonization and being next to America, it was easier to be the same.