Left-side driving appears to date to the Middle Ages. People then traveled on the left side of the road for several reasons: Most folks, like now, were right-handed. They found it easier to wield a weapon against an enemy or welcome a friend with the right hand, which they preferred to keep on the passing side of the road. It was also safer to dismount a horse to the left while wearing a left-slung sword. And it was more advisable to dismount and mount, which can only be done from the left by right-handed people, on the outside, therefore left side of the road, rather than on the inside, right side, in the midst of oncoming traffic!
This left-side driving habit was transported from feudal England to the European continent and, later, to the far reaches of the British Empire. But at the time of both the French and American revolutions, folks felt it best to eschew everything from their monarchical pasts. In France, aristocrats and noblemen of the ancien régime had traveled the road on the left, forcing the peasantry to the right. From the outset of the Revolution, these former gentry found they could blend more easily with the general population by joining the right-moving crowd.
Simultaneously, in both France and the US in the late 1700s, teamsters began hauling farm products long distances in big wagons pulled by several pairs of horses. The wagons were not equipped with seats. So drivers sat on the left rear horse, using their right arm to lash the four-legged members of the team. In contrast to their feudal forebears, the teamsters preferred to pass on the right so they could better see and stay clear of the wheels of oncoming vehicles.
Emperor Napoleon Bonaparte spread rightism throughout conquered Europe during his early 19th century campaigns. This left England, the Austro-Hungarian Empire, and Portugal – the countries that resisted or were left untouched by Napoleon – to continue the old habit of left-side driving. This right-side/left-side division remained in Europe for more than 100 years, until after the First World War. At that point, the rest of continental Europe shifted to the right. Only Britain and the countries of the former British Empire kept to the left. But even Canada eventually switched in order to make border crossing with the US less complicated.
Now, in an odd and unprecedented move, Samoa is going from right back to left. Bon courage, les Samoans!
View of Upolu, Independent Samoa, by Kronocide, courtesy of Wikimedia Commons.
This print, entitled "The knight of the woeful countenance going to extirpate the National Assembly," shows Edmund Burke as Don Quixote, wearing armor, carrying lance and shield labeled "Shield of Aristocracy and Despotism," riding a donkey, emerging from the doorway to the "Dodsley Bookseller" the publisher of Burke's "Reflections on the French Revolution" which hangs from the horn of the saddle. The head of the donkey has a human face and wears the triple-tiered crown of the pope; depicted on the shield are scenes of torture and death, and a view of the Bastille. Found on Wikimedia Commons.
Driving on the left side in Australia, taken on 11.03.2006 on the Great Ocean Road (near Lorne) in Victoria (Australia), courtesy of Free Software Foundation, and found on Wikimedia Commons.