Historically, most travelers on foot or horseback kept to the left so their dominant right hand stayed free to greet a passerby or draw a weapon if needed. Driving on the right spread largely through French and later American influence — Napoleon's armies enforced right-hand travel across the territories they controlled, and the custom stuck even after empires faded. Countries that were British colonies, like India, Japan, and Australia, mostly kept the original left-hand convention, which is why the split today roughly traces old colonial and trade influence rather than anything about the cars themselves.