During the age of colonisation and empires, the conquerors usually imposed their driving customs on their colonies. Many countries which drive on the left today do so because of the fact that they were once part of the British Empire, notably Australia, New Zealand and several African countries.
Napoleon's conquests in Europe forced most of the Continent to drive on the right to conform with French practice. During the second World War, Austria, Czechoslovakia, Hungary, and the Channel Islands changed sides from left to right to conform to German practice.
The most recent example of a European country changing sides was Sweden. By the 1950s, the Swedes felt increasing pressure to change sides to conform with the rest of Europe. The problem with left-hand driving in Sweden was, of course, that its neighbours drove on the right side. There are many small roads, without border posts, leading into Norway creating severe problems when drivers forgot which country they were in.
A referendum on the question of introducing right-hand driving was held in 1955, with the vote being 82.9 per cent against and only 15.5per cent in favour of the conversion. However, in the best traditions of European democracy the Swedish parliament decided to ignore this and passed a law to convert to right-hand driving. The change took place in the early Sunday morning at 5:00 on September 3rd 1967.
All traffic with private motor driven vehicles was prohibited four hours before and one hour after the conversion. In some cities there was no private traffic for 24 to 29 hours. The changeover went smoothly and the Swedes have never looked back.
Many other countries have made the change over the years including the Panama Canal Zone in the 1920s. Despite initial predictions of carnage on the roads accident levels actually decreased in the period following it.
Nigeria changed in the 1960s for possibly the most bizarre of reasons. The country had decided to decimalise its currency and decided that since most decimal currency countries drove on the right they should too.
Finally there is the case of Myanmar (Burma), which was a British colony until 1948, and drove on the left until 1970, when it changed sides. It is said that the ruler of the country, Ne Win, consulted a soothsayer or interpreted a dream to mean that all traffic should keep to the right.