Left- and right-hand traffic

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.