I see that Samoa has switched over from driving on the right to driving on the left. It is the natural thing. Grooves in Roman roads show that the Romans drove their chariots on the left. It is said to be more convenient to have one's sword arm free to meet oncoming traffic. Research in 1969 by J. J. Leeming showed countries driving on the left have a lower collision rate than countries driving on the right. It has been suggested this is partly because humans are more commonly right-eye dominant than left-eye dominant. In left-hand traffic, the predominantly better-performing right eye is used to monitor oncoming traffic and the driver's wing mirror. In right-hand traffic, oncoming traffic and the driver's wing mirror are handled by the predominantly weaker left eye. In addition, it has been argued that left sided driving is safer for elderly people given the likelihood of them having visual attention deficits on the left side and the need at intersections to watch out for vehicles approaching on the near-side lane.
The reason that Samoa changed was to be able to import cheaper right-hand drive cars from nearby Australia and New Zealand. It is mostly former British colonies that continue to drive on the left, but notably, Japan and Indonesia drive on the left, perhaps this is why Japanese car firms have settled so easily in the UK - Nissan, Toyota
and Honda all have large factories here. The country with the largest population that drives on the left is India. The former Portuguese colony, Mozambique, drives on the left, even though Portugal switched to the right in 1928. Of former British colonies, Canada, Gambia, Ghana, Nigeria, Sierra Leone and the United States have all switched to right hand driving; Canada switched in 1923, but the USA changed sometime in the 1700s. It does not seem to have been under the influence of Napoleon, but because of the use of heavy waggons drawn by several teams of horses The Postilion sat on the left rearmost horse and preferred to have wagons passing him in the other direction go by on the left. Talking of Napoleon, in France, along the 350 metres (380 yd) of Avenue du Général Lemonnier in Paris, which connects the Pont Royal to the Rue de Rivoli, traffic drives on the left.
Iceland switched traffic from left to right at 06:00 on Sunday 26 May 1968; the only injury from the changeover was a boy on a bicycle who broke his leg
Control of driving side is often dictated by dictators or conquerors - Mussolini dictated that Italy should switch, Hitler changed both Czechoslovakia and Austria. Korea and Taiwan changed back when the Japanese left. Right-hand traffic was introduced in the Philippines on the last day of the Battle of Manila, 10 March 1945, to facilitate American troop movements. But Hong Kong has remained on the left since it became part of China. Burma (a former British colony now known there but almost nowhere else as Myanmar) switched in 1970 on the advice of a wizard. The US Virgin Islands (a former Danish colony) drive on the left as do the US Sovereign bases in the UK.
Oh and the standard gauge on British railways of 4 feet 7 inches was determined as the width of two horses backsides.