Left-hand traffic (LHT) and right-hand traffic (RHT) are the practices, in bidirectional traffic, of keeping to the left side or to the right side of the road, respectively. They are fundamental to traffic flow, and are sometimes called a "rule of the road". The terms right- and left-hand "drive" refer to the position of the driver and the steering wheel in the vehicle and are, in automobiles, the reverse of the terms right- and left-hand "traffic". The rule also includes where on the road a vehicle is to be driven, if there is room for more than one vehicle in one direction, and the side on which the vehicle in the rear overtakes the one in the front. For example, a driver in an LHT country would typically overtake on the right of the vehicle being overtaken. RHT is used in 165 countries and territories, while 75 countries use LHT, which account for about a sixth of the world's land area, a quarter of its roads, and about a third of its population. While many of the countries using LHT were part of the British Empire, others such as Indonesia, Japan, Nepal, Bhutan, Macau, Thailand, and Mozambique were not. Sweden and Iceland, which have used RHT since September 1967 and late May 1968 respectively, previously used LHT. All of the countries that were part of the French Colonial Empire adopted RHT. Between 1919 and 1986, 34 of the LHT territories switched to RHT. In 1919, 104 of the world's territories were LHT and an equal number were RHT. Switches have been motivated by factors such as changes in political administration, a desire for uniformity within a country or with neighboring states, and availability and affordability of vehicles. In LHT, traffic keeps left and cars usually have the steering wheel on the right (RHD: right-hand drive) and roundabouts circulate clockwise. RHT is the opposite: traffic keeps right, the driver usually sits on the left side of the car (LHD: left-hand drive), and roundabouts circulate counterclockwise. In most countries, rail traffic follows the handedness of the roads; but many of the countries that switched road traffic from LHT to RHT did not switch their trains. Boat traffic on bodies of water is RHT, regardless of location. Boats are traditionally piloted from the starboard (right) side, unlike RHT road traffic vehicles, to facilitate priority to the right.
Background
To prevent traffic collisions, all societies adopted a rule that required traffic travelling in opposite directions to yield to the left or to yield to the right. Traffic approaching one another head-on would not collide if the drivers of both yielded to the same side. Why some societies preferred or selected one over the other is not clear. Some claims relate to the hand selected for holding horse reins, for self-defense with weapons, or controlling horses with whips. Academic Chris McManus notes that writers have stated that in 1300, Pope Boniface VIII directed pilgrims to keep left; others suggest that he directed them to keep to the right, and there is no documented evidence to back either claim.
Geographical areas
Africa The British Empire introduced LHT in the East Africa Protectorate (present-day Kenya), the Protectorate of Uganda, Tanganyika (formerly part of German East Africa; present-day Tanzania), Rhodesia (present-day Zambia and Zimbabwe), Eswatini, the Cape Colony (part of present-day South Africa) and Lesotho, as well as in British West Africa (present-day Ghana, Gambia, Sierra Leone and Nigeria); former British West Africa, however, has now switched to RHT, as all its neighbours, which are mostly former French territories, use RHT. South Africa introduced LHT in former German South West Africa, present-day Namibia, after the end of World War I. Sudan, formerly part of Anglo-Egyptian Sudan, switched to RHT in 1973. Most of its neighbours were RHT countries, with the exception of Uganda and Kenya, but since the independence of South Sudan in 2011, all of its neighbours drive on the right (including South Sudan, despite its land borders with two LHT countries). Although Portugal switched to RHT in 1928, its colony of Mozambique remained LHT because it has land borders with former British colonies (with LHT). France introduced RHT in French West Africa and the Maghreb, where it is still used. Countries in these areas include Mali, Mauritania, Ivory Coast, Burkina Faso, Benin, Niger, Morocco, Algeria, and Tunisia. Other French former colonies that are RHT include Cameroon, Central African Republic, Chad, Djibouti, Gabon, and the Republic of the Congo. Rwanda and Burundi are RHT but are considering switching to LHT (see "Potential future shifts" section below).
Americas
United States In the late 18th century, right-hand traffic started to be introduced in the United States based on teamsters' use of large freight wagons pulled by several pairs of horses and without a driver's seat; the (typically right-handed) postilion held his whip in his right hand and thus sat on the left rear horse, and therefore preferred other wagons passing on the left so that he would have a clear view of other vehicles. The first keep-right law for driving in the United States was passed in 1792 and applied to the Philadelphia and Lancaster Turnpike. Massachusetts formalized RHT in 1821. However, the National Road was LHT until 1850, "long after the rest of the country had settled on the keep-right convention". Today the United States is RHT except the United States Virgin Islands, which is LHT like many neighbouring islands. Some special-purpose vehicles in the United States, like certain postal service trucks, garbage trucks, and parking-enforcement vehicles, are built with the driver's seat on the right for safer and easier access to the curb. A common example is the Grumman LLV, which is used nationwide by the US Postal Service and by Canada Post.
Other countries in the Americas
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