We didn't always travel this way. In the Middle Ages, paranoid knights rode on the left because it was easier to defend themselves from attackers coming the other way. (A clear bias against southpaws: The position gave right-handed swordsmen the edge.) By 1300, Pope Boniface VIII wrote the practice into law.

But things changed in the 18th century when American and French wagon drivers started using up to six horses to pull heavy loads. Wagons didn’t have seats, so right-handed drivers usually perched on the back-left horse to avoid cracking the whip across their bodies (another bias against southpaws!). In order to get a clear view of oncoming traffic, they traveled down the right side of the road. When Napoleon gobbled up most of Europe decades later, he forced conquered countries to convert. A century later, Ford’s Model-T convinced even more nations to make the switch.

This story originally appeared in the July 2014 issue of mental_floss magazine. Subscribe to our print edition here, and our iPad edition here.