�We do not know which side of the road the Romans drove on. There just isn�t enough evidence either way,� said Catherine Jones, expert on Roman archaeology at the British Museum, in October. But Bryn Walters of the Association for Roman Archaeology says he does know, and his argument is very persuasive. They drove on the left.

Though the straight roads built by the Roman empire still define the routes of many modern roads in Europe and the Middle East, they have been rebuilt so many times over the past 2,000 years that little original material remains. And since Latin literature did not go in for stories about the lives of cart-drivers, which side they drove on was unknown � until this year, when Walters found the track into the old Roman quarry at Blunsdon Ridge.

The track was only used for bringing stone from the quarry to a major Roman temple being built on the nearby ridge (near Swindon in England), and then fell out of use, so it is very well preserved. And since the carts went in empty and came out laden with stone, the ruts on one side of the road are much deeper than they are on the other. The conclusion: Romans drove on the left.

Why they chose to drive on the left remains a mystery. Perhaps it dated back to earlier times when travellers on horseback preferred to keep to the left when encountering strangers, so that their sword-hand was free in case of a problem. (Most people everywhere are right-handed.) But at least as far back as Roman times, it seems clear, wheeled traffic in most of Europe and the Mediterranean world kept to the left.

So why does all of Europe (except the British Isles), all of the Western hemisphere (except some former British possessions in the Caribbean), and all of the Middle East drive on the right? That seems to be Napoleon�s fault.

In the long Dark Age after the fall of the Roman empire, and even in the Middle Ages, there would not have been much need for the drive-left rule, since what little wheeled traffic there was travelled mostly on narrow tracks. But when you met somebody else on those narrow tracks both parties had to veer either left or right, and in that sense the Roman rule seems to have survived: mostly, people swung out to the left.

In early modern Europe, with the volume of road traffic rising steadily, the old Roman custom of driving on the left was the likelier candidate to become the new legal standard � as it did in Britain, in Sweden, and in various other places that Napoleon never reached. But wherever the French emperor�s armies invaded, they imposed a new rule: driving on the right. Why?

Napoleon never said, and subsequent historians have mumbled half-explanations about his need to impose discipline on European road traffic so that his armies could get through. But why did he go against the existing custom, frequently ignored though it undoubtedly was, and impose driving on the right? Probably precisely because driving on the left was the custom.

Napoleon was a product of the French Revolution (however far he was from the ideals of the original revolutionaries), and the whole ethos of the revolution was about the breaking of the old rules and the creation of a new, rational world. The year 1789 became Year One of the new era, and even the months were renamed.

Driving right is no more rational than driving left, but it is more �revolutionary.� That would have appealed to Napoleon � and since his armies went everywhere from Russia to Spain, almost all of mainland Europe ended up driving on the right. (The Swedes finally gave up and switched a couple of decades ago.)

That meant that all the Spanish and Portuguese colonies in the Americas also ended up driving on the right. So did the infant United States, presumably because it felt closer to fellow revolutionaries in France than to the former British oppressor.

Even British North America (now Canada) eventually switched to the right, because it made no sense to drive on different sides of the road along the world�s longest land frontier. And the Middle East drives right because the Ottoman empire, which used to rule most of the region, was heavily under the influence of the right-driving French and Germans at the critical time when its army laid down formal traffic rules in the latter half of the 19th century.

But in most of sub-Saharan Africa, except for former French colonies, people drive on the left because of the British influence. They do the same in almost all the countries from Pakistan and India to Australia and New Zealand; only ex-French Indochina and the Philippines, an ex-U.S. colony, drive right.

Even Indonesia (which was briefly occupied by the British two centuries ago) and Thailand (which was never colonized at all) drive on the left. So does Japan, though nobody seems to know whether this is due to 19th-century British influence, or whether it is as deeply rooted in Japan as it was in post-Roman Europe.

Korea now drives right, but only because it passed directly from Japanese colonial rule to American (and Russian) influence at the end of the Second World War. And I just don�t know why China now drives right, or if it ever drove on the left.

The metric system, the other great standardization that we inherited from the French revolution, has become the global norm. Only the United States (and Britain, at least where road signs are concerned) still stick with the old English measures. But by making the �wrong� side his standard, Napoleon has left us a world permanently divided between countries that drive right (about 3.5 billion people) and those that drive left (about 2.5 billion).

Napoleon was a great admirer of the Roman army. If only he had known which side the Romans drove on, it might all have been different.