Which side to drive on has been an issue since people first hitched horses to carts.
While the ancient civilizations of the Mediterranean quickly developed rules, whatever uniformity there might have been in the ancient world was lost when the Roman Empire fell.
After that, things went left or right according to the whim of locals, and the locals didn't keep good records.
Australian university lecturer Peter Kincaid sheds light on the issue in his book The Rule of the Road: An International Guide to History and Practice .
In the Middle Ages armed walkers or horsemen preferred to travel on the left side of the road to keep their weapon-bearing sides free and to favor their best eye.
Anybody leading a heavy wagon or a cart prefers to sit on the left side.
So, says Kincaid, it's easy to see how, in a samurai-dominated Japan or a knight-controlled England, the keep-left rule became cemented enough to resist being overturned by right-side (wagon-based) commercial interests.
Countries that drive on the left today "account for only about a third of the world's population and a sixth of its motor vehicles," says Kincaid.
Since 1913 there have been 13 right-left changes among nations, but 45 left-right changes.
Does this mean someday the whole world will drive on the right? The trend is "unlikely to go much further," says Kincaid, "unless Britain and Japan change. There is no prospect of their doing so."
Kincaid cites studies that seem to show the left side is the best side for driving -- psychobiologically. Choosing sides based on science and safety would put us on the left.
Which raises the question: What if the left-siders are, as it were, right?