Left-hand traffic (LHT) and right-hand traffic (RHT) are the practices, in bidirectional traffic, of keeping to the left side and to the right side of the road, respectively. They are fundamental to traffic flow, and are sometimes called the 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 the 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, mainly in the Americas, Continental Europe, northern Africa and mainland Asia (except South Asia), while 75 countries use LHT, which account for about a sixth of the worlds land area, a quarter of its roads, and about a third of its population. In 1919, 104 of the worlds territories were LHT and an equal number were RHT. Between 1919 and 1986, 34 of the LHT territories switched to RHT.
Many of the countries using LHT were part of the British Empire, while others, including Indonesia, Japan, Nepal, Bhutan, Macao, Thailand, Mozambique, Suriname, United States Virgin Islands, Sweden and Iceland (which use RHT since September 1967 and late May 1968 respectively), were not. Most of the countries that were part of the French colonial empire adopted RHT.
Historical switches of traffic handedness have often been motivated by factors such as changes in political administration, a desire for uniformity within a country or with neighboring states, or 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 rivers is RHT, regardless of location. Boats are traditionally piloted from the starboard side to facilitate priority to the right.