About a quarter of the world drives on the left, and the countries that do are mostly old British colonies. This strange quirk perplexes the rest of the world; but there is a perfectly good reason.

In the past, almost everybody travelled on the left side of the road because that was the most sensible option for feudal, violent societies. Since most people are right-handed, swordsmen preferred to keep to the left in order to have their right arm nearer to an opponent and their scabbard further from him. Moreover, it reduced the chance of the scabbard (worn on the left) hitting other people.

Furthermore, a right-handed person finds it easier to mount a horse from the left side of the horse, and it would be very difficult to do otherwise if wearing a sword (which would be worn on the left). It is safer to mount and dismount towards the side of the road, rather than in the middle of traffic, so if one mounts on the left, then the horse should be ridden on the left side of the road.

In the late 1700s, however, teamsters in France and the United States began hauling farm products in big wagons pulled by several pairs of horses. These wagons had no driver's seat; instead the driver sat on the left rear horse, so he could keep his right arm free to lash the team. Since he was sitting on the left, he naturally wanted everybody to pass on the left so he could look down and make sure he kept clear of the oncoming wagon’s wheels. Therefore he kept to the right side of the road.

In addition, the French Revolution of 1789 gave a huge impetus to right-hand travel in Europe. The fact is, before the Revolution, the aristocracy travelled on the left of the road, forcing the peasantry over to the right, but after the storming of the Bastille and the subsequent events, aristocrats preferred to keep a low profile and joined the peasants on the right. An official keep-right rule was introduced in Paris in 1794, more or less parallel to Denmark, where driving on the right had been made compulsory in 1793.

Later, Napoleon's conquests spread the new rightism to the Low Countries (Belgium, the Netherlands and Luxembourg), Switzerland, Germany, Poland, Russia and many parts of Spain and Italy. The states that had resisted Napoleon kept left – Britain, the Austro-Hungarian Empire and Portugal. This European division, between the left- and right-hand nations would remain fixed for more than 100 years, until after the First World War.

Although left-driving Sweden ceded Finland to right-driving Russia after the Russo-Swedish War (1808-1809), Swedish law – including traffic regulations – remained valid in Finland for another 50 years. It wasn’t until 1858 that an Imperial Russian decree made Finland swap sides.

The trend among nations over the years has been toward driving on the right, but Britain has done its best to stave off global homogenisation. With the expansion of travel and road building in the 1800s, traffic regulations were made in every country. Left-hand driving was made mandatory in Britain in 1835. Countries which were part of the British Empire followed suit. This is why to this very day, India, Australasia and the former British colonies in Africa go left. An exception to the rule, however, is Egypt, which had been conquered by Napoleon before becoming a British dependency.

Although Japan was never part of the British Empire, its traffic also goes to the left. Although the origin of this habit goes back to the Edo period (1603-1867) when Samurai ruled the country, it wasn’t until 1872 that this unwritten rule became more or less official. That was the year when Japan’s first railway was introduced, built with technical aid from the British. Gradually, a massive network of railways and tram tracks was built, and of course all trains and trams drove on the left-hand side. Still, it took another half century till in 1924 left-side driving was clearly written in a law.

When the Dutch arrived in Indonesia in 1596, they brought along their habit of driving on the left. It wasn't until Napoleon conquered the Netherlands that the Dutch started driving on the right. Most of their colonies, however, remained on the left as did Indonesia and Suriname.

In the early years of English colonisation of North America, English driving customs were followed and the colonies drove on the left. After gaining independence from England, however, they were anxious to cast off all remaining links with their British colonial past and gradually changed to right-hand driving. (Incidentally, the influence of other European countries’ nationals should not be underestimated.) The first law requiring drivers to keep right was passed in Pennsylvania in 1792, and similar laws were passed in New York in 1804 and New Jersey in 1813.

Despite the developments in the US, some parts of Canada continued to drive on the left until shortly after the Second World War. The territory controlled by the French (from Quebec to Louisiana) drove on the right, but the territory occupied by the English (British Columbia, New Brunswick, Nova Scotia, Prince Edward Island and Newfoundland) kept left. British Columbia and the Atlantic provinces switched to the right in the 1920s in order to conform with the rest of Canada and the USA. Newfoundland drove on the left until 1947, and joined Canada in 1949.

In Europe, the remaining left-driving countries switched one by one to driving on the right. Portugal changed in 1920s. The change took place on the same day in the whole country, including the colonies. Territories, however, which bordered other left-driving countries were exempted. That is why Macau, Goa (now part of India) and Portuguese East Africa kept the old system. East Timor, which borders left-driving Indonesia, did change to the right though, but left-hand traffic was reintroduced by the Indonesians in 1975.

In Italy the practice of driving on the right first began in the late 1890s. The first Italian Highway Code, issued on the 30th of June 1912, stated that all vehicles had to drive on the right. Cities with a tram network, however, could retain left-hand driving if they placed warning signs at their city borders. The 1923 decree is a bit stricter, but Rome and the northern cities of Milan, Turin and Genoa could still keep left until further orders from the Ministry of Public Works. By the mid-1920s, right-hand driving became finally standard throughout the country. Rome made the change on the 1 of March 1925 and Milan on the 3rd of August 1926.

Up till the 1930s Spain lacked national traffic regulations. Some parts of the country drove on the right (e.g. Barcelona) and other parts drove on the left (e.g. Madrid). On the 1st of October 1924 Madrid switched to driving on the right.

The break-up of the Austro-Hungarian Empire caused no change: Czechoslovakia, Yugoslavia and Hungary continued to drive on the left. Austria itself was something of a curiosity. Half the country drove on the left and half on the right. The dividing line was precisely the area affected by Napoleon's conquests in 1805.

When Germany annexed Austria in 1938, Hitler ordered that the traffic should change from the left to the right side of the road, overnight. The change threw the driving public into turmoil, because motorists were unable to see most road signs. In Vienna it proved impossible to change the trams overnight, so while all other traffic took to the right-hand side of the road, the trams continued to run on the left for several weeks. Czechoslovakia and Hungary, one of the last states on the mainland of Europe to keep left, changed to the right after being invaded by Germany in 1939.

Meanwhile, the power of the right kept growing steadily. American cars were designed to be driven on the right by locating the drivers' controls on the vehicle's left side. With the mass production of reliable and economical cars in the United States, initial exports used the same design, and out of necessity many countries changed their rule of the road.

Gibraltar changed to right-hand traffic in 1929 and China in 1946. 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. Pakistan also considered changing to the right in the 1960s, but ultimately decided not to do it. The main argument against the shift was that camel trains often drove through the night while their drivers were dozing. The difficulty in teaching old camels new tricks was decisive in forcing Pakistan to reject the change. Nigeria, a former British colony, had traditionally been driving on the left with British imported right-hand-drive cars, but when it gained independence, it tried to throw off its colonial past as quick as possible and shifted to driving on the right.

After the Second World War, left-driving Sweden, the odd one out in mainland Europe, felt increasing pressure to change sides in order to conform with the rest of the continent. The problem was that all their neighbours already drove on the right side and since there are a lot of small roads without border guards leading into Norway and Finland, one had to remember in which country one was.

In 1955, the Swedish government held a referendum on the introduction of right-hand driving. Although no less than 82.9% voted “no” to the plebiscite, the Swedish parliament passed a law on the conversion to right-hand driving in 1963. Finally, the change took place on Sunday, the 3rd of September 1967, at 5 o’clock in the morning.

All traffic with private motor-driven vehicles was prohibited four hours before and one hour after the conversion, in order to be able to rearrange all traffic signs. Even the army was called in to help. Also a very low speed limit was applied, which was raised in a number of steps. The whole process took about a month. After Sweden's successful changeover, Iceland changed the following year, in 1968.

In the 1960s, Great Britain also considered changing, but the country’s conservative powers did everything they could to nip the proposal in the bud. Furthermore, the fact that it would cost billions of pounds to change everything round wasn’t much of an incentive… Eventually, Britain dropped the idea. Today, only four European countries still drive on the left: the United Kingdom, Ireland, Cyprus and Malta.