Map of the world showing the driving directions for all countries and any changes that have occurred, beginning with Finland’s change in 1858
     drives on right      drove on left, now drives on right      drives on left      drove on right, now drives on left      had different rules of the road within borders, now drives on right

[Map from Right- and left-hand traffic]

While working at the National College of Ireland, I got into a lunchtime discussion about driving, in particular, about why some countries, such as Ireland, drive on the left and others, such as the US, drive on the right. None of us could even say with much certainty how many people in the world did it one way or the other. Seemingly simple questions led to many jokes, confident pronouncements, and further questions about changeovers, what boats and planes do, and so on.

As I began to look into it more, I realized this was yet another example of how simple questions can lead to interesting inquiries without end. One of the most interesting sources I came across was a website, Which side of the road do they drive on?, edited by Brian Lucas, and based in large part on The Rule of the Road: An International Guide to History and Practice by Peter Kincaid. Here’s an excerpt from the site:

The side of the road one drives on seems to be one of those mundane details of everyday life which people take for granted and never bother to write down. In his book, Kincaid comes up with a blank: “I have been unable to discover any firm evidence as to what the rule of the road was in any part of the ancient civilizations in Greece, Rome, or Assyria. It seems inconceivable that there was not one.” We have found, however, evidence that the ancient Romans drove on the left.

In late 1998, the remains of a Roman quarry was discovered at Blunsdon Ridge, near Swindon, England. It is one of the largest and best-preserved Roman quarries known. Ruts in the road leading to this quarry are much deeper on one side of the road than on the other. If it can be assumed that the side of the road with deeper ruts was the side used by loaded carts leaving the quarry, while the side with shallow ruts indicates empty carts arriving, then we can conclude that at this particular location, at least, the Romans drove on the left. (Sources: a web page in the SwindonWeb Local News Archives for October 1998 which has since disappeared, and an article by Gwynne Dyer, Is driving on the right right or wrong?, from 1999.)

Another piece of evidence comes from a Roman coin. Robert Pease writes that he has seen a picture of a denarius from between 50 BC and 50 AD showing two horsemen riding past each other, right shoulder to right shoulder (i.e. each keeping to the left side of the road).

It was amazing to me to learn how this simple practice connects with our physical characteristics, modes of transportation, revolution and empire, warfare, commerce, and many other aspects of daily life.