Driving vehicles on the left was a natural outcome of a feudal, violent society. Swordsmen, mounted on horses, kept left of their opponents, with the sword held on the left hip, giving them the best advantage with their dominant hand. Mounting and dismounting was from the left of the horse to keep the sword clear. Dismounting left was safer when you were riding to the left of any traffic in congested areas.
In the late 1700's, teamsters in France and the American colonies, hauling farm products in large wagons with teams of horses, switched lanes, so to speak. Mounting from the left, they were seated on the left and wanted to have a better visual to steering their wagon wheels clear of opposing traffic. Since then, nations have had to decide...left or right? The French Revolution caused a huge swing in Europe toward right handed driving, but Britain and its' colonies were mandated to drive on the left. Politics, practicality, tradition or otherwise, 35% of countries drive on the left and with the infrastructure in place for that, it would be quite the expense to change over now.
It really only becomes an issue when you decide to drive in one of these countries that is opposite to how you have been trained. Jim's first Father's Day was quite memorable as we were involved in a head on collision with a driver on a mountain road who came around the corner on the wrong side. He had just landed in the US that morning from England.
When we arrived on the Isle of Man, we realized, with me now in a wheelchair, that our best option to see the most of the island would be with a car. Jim navigated our family safely around the island for four days with a stick shift on his left and traffic on his right. Most intersections in the countryside were roundabouts and our pleasant British accented GPS helped with navigation immensely. At one point, Jim declared, "This is just wrong." Getting back to Germany and then the US he is happy to be driving "right" again.