system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Is there a reason/story behind why we drive on different sides of the road in different countries?
gpt
Actually it's interesting. Back when most travel was on foot, and traveling was a dangerous venture, and most people were right handed, people traveled on the left side of the roads so that their "sword hand" would be on the right.  


Now Napoleon changed things up a little bit. He was left-handed so he insisted on marching on the right, forcing all his troops to do the same. At the time, nearly ALL of Europe was under Napoleon's control so the change was made across the entire continent (except Britain of course) 


America definitely wanted to get rid of whatever British traditions it could, so they were all for using the right side of the road. (Additionally there was a practical purpose of drivers of carriages being to whip all horses from the rear-left position of the vehicle) So when America led the world in developing and producing the automobile and developed it for right-side driving, and the rest of the world bought American cars, well we have the modern roadway system.


So basically the only people who drive on the left still are Britain and some of its non-European (previous) crown colonies.  (as some have pointed out Japan is a notable exception but also tied to British reasons)