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 referred to as 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 extends to where on the road a vehicle is to be driven, if there is room for more than one vehicle in the one direction, as well as 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, with the remaining 75 countries and territories using LHT.

Countries that use left-hand traffic account for about a sixth of the world's land area, with about a third of its population, and a quarter of its roads. In 1919, 104 of the world's 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 that adopted LHT were formerly part of the British Empire, although some, such as Indonesia, Japan, Mozambique, Nepal, Suriname, Sweden (RHT since 1967), Thailand, and the city of Macau were not. Similarly, many of the countries that were a part of the French colonial empire adopted RHT.

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 of this: 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, although many of the countries that switched road traffic from LHT to RHT did not switch their trains. Boat traffic on rivers is always RHT, regardless of location. Boats are traditionally piloted from the starboard side to facilitate priority to the right.