The day Sweden switched from left to right in 1967 Kungsgatan
Why do some countries drive on the right and some on the left? It turns out that it's just as complicated as you might imagine. If you are sitting comfortably, here is the history behind it all.
In times gone by, there was no such thing as describing someone as a left-handed person. Unless you'd lost it, your right hand was your sword hand. Therefore, when travelling, it was common sense to stay on the left of the road, so you could better defend yourself from whatever was coming the other way. This common sense practice became widespread, and in 1300 AD even the Pope was advising pilgrims to travel on the left.
In 1998, overly excited archaeologists in Swindon found a preserved track of Roman road leading from an ancient quarry. Grooves worn into the road on the left side (as you exited the quarry) were much deeper than on the right. The heavy, fully loaded carts leaving the quarry were then clearly driving on the left, indicating that left-hand traffic in Britain goes all the way back to the Roman era. This isn't surprising, as the Romans are known to have been very strict with their congestion and traffic laws. Eventually, in 1773, at the height of the first British Empire, increasingly heavy horse-drawn traffic forced the government to formalise the common understanding of 'keep left' as law. No one complained, because a traffic jam of horse-drawn vehicles smells.
So why don't they drive on the left in mainland Europe? Well, until the French Revolution, it's thought they did. Before the revolution, it was actually dangerous for French peasants to walk on the left, with their backs to oncoming traffic, because the aristocracy would rather run them over than slow down. After the revolution, with almost all of the aristocrats being beheaded, no one wanted to drive on the left and look like one. Later, when Napoleon conquered Spain, Italy and most of Germany, he enforced what had become a right-hand traffic law. Later still, Hitler enforced this law in places like Czechoslovakia, Poland and Hungary. After a while, with most of the continent now driving on the right, other countries, such as Sweden in 1967, made the change so it would be simpler for people travelling abroad (see image above). Everyone agreed that it was nice of them.
In America, the switch from left to right was due to the use of large freight wagons in the 1700s. These wagons, pulled by several pairs of horses, had no driver's seat (or air conditioning). Instead, a postilion sat on the back left horse, with his whip in his right hand. In this position, it was easier for the postilion to keep the wagon to the right, giving him a better view of the road and oncoming traffic.
For obvious reasons, whether it's right-hand or left-hand traffic, it's important to have an established system (so important it's in a Geneva Convention). However, there aren't any obvious benefits one has over the other. There have been studies that show a significantly lower crash rate in countries that drive on the left, but none that conclusively proved the traffic system to be the reason. For the sake of the 65 percent of the world's population that drive on the right, let's hope it's not.