Left- and right-hand traffic

Countries by handedness of traffic, c. 2018
  Left-hand traffic
  Right-hand traffic
Change of traffic directions at the Thai–Lao Friendship Bridge

The terms left-hand traffic (LHT) and right-hand traffic (RHT) refer to the practice, in bidirectional traffic, to keep to the left side or to the right side of the road, respectively. This is so fundamental to traffic flow that it is sometimes referred to as the rule of the road.[1]

RHT is used in 163 countries and territories, with the remaining 78 countries and territories using LHT. Countries that use LHT account for about a sixth of the world's area with about 35% of humanity and a quarter of its roads.[2] In 1919, 104 of the world's territories were LHT and an equal number were RHT. From 1919 to 1986, 34 of the LHT territories switched to RHT.[3]

Many of the countries with LHT were formerly part of the British Empire. In addition, Japan, Indonesia, Thailand, Suriname and other countries have retained the LHT tradition. Conversely, many of the countries with RHT were formerly part of the French colonial empire or, in Europe, were subject to French rule during the Napoleonic conquests.

For rail traffic, LHT predominates in Western Europe (except Germany, Austria, Spain and the Netherlands), Latin America, and in countries formerly in the British and French Empires, whereas North American and central and eastern European train services operate RHT.

According to the International Regulations for Preventing Collisions at Sea, water traffic is RHT. For aircraft the US Federal Aviation Regulations suggest RHT principles, both in the air and on water.[4]

In healthy populations, traffic safety is thought to be the same regardless of handedness, although some researchers have speculated that LHT may be safer for ageing populations[5] since humans are more commonly right-eye dominant than left-eye dominant.[6]

History

Left-hand traffic in Vienna, Austria circa 1930.

Ancient Greek, Egyptian, and Roman troops kept to the left when marching.[7] In 1998, archaeologists found a well-preserved double track leading to a Roman quarry near Swindon, in southern England. The grooves in the road on the left side (viewed facing down the track away from the quarry) were much deeper than those on the right side, suggesting LHT, at least at this location, since carts would exit the quarry heavily loaded, and enter it empty.[8]

The first reference in English law to an order for LHT was in 1756, with regard to London Bridge.[9]

Hong Kong drives on the left.
Mainland China drives on the right.

Some historians, such as C. Northcote Parkinson, believed that ancient travellers on horseback or on foot generally kept to the left, since most people were right-handed. If two men riding on horseback were to start a fight, each would edge toward the left.[7] In the year 1300, Pope Boniface VIII directed pilgrims to keep left.[7]

In the late 1700s, traffic in the United States was RHT based on teamsters' use of large freight wagons pulled by several pairs of horses. The wagons had no driver's seat, so the (typically right-handed) postilion held his whip in his right hand and thus sat on the left rear horse. Seated on the left, the driver preferred that other wagons pass him on the left so that he could be sure to keep clear of the wheels of oncoming wagons.[10]

In France, traditionally foot traffic had kept right, while carriage traffic kept left. Following the French Revolution, all traffic kept right.[9] Following the Napoleonic Wars, the French imposed RHT on parts of Europe.[citation needed] During the colonial period, RHT was introduced by the French in New France, French West Africa, the Maghreb, French Indochina, the West Indies, French Guiana and the Réunion, among others.

Meanwhile, LHT was introduced by the British in parts of Canada (Atlantic Canada and British Columbia), Australia, New Zealand, the East Africa Protectorate (now Kenya, Tanzania and Uganda), British India, Rhodesia and the Cape Colony (now Zambia, Zimbabwe and South Africa), British Malaya (now Malaysia, Brunei and Singapore), British Guiana, and British Hong Kong. LHT was also introduced by the Portuguese Empire in Portuguese Macau, Colonial Brazil, Portuguese Timor, Portuguese Mozambique, and Portuguese Angola.

The first keep-right law for driving in the United States was passed in 1792 and applied to the Philadelphia and Lancaster Turnpike.[11] New York formalized RHT in 1804, New Jersey in 1813 and Massachusetts in 1821.[12]

In the early 1900s some countries including Canada, Spain, and Brazil had different rules in different parts of the country. During the 1900s many countries standardised within their jurisdictions, and changed from LHT to RHT, mostly to conform with regional custom. Currently nearly all countries use one side or the other throughout their entire territory. Most exceptions are due to historical considerations and/or involve islands with no road connection to the main part of a country. China is RHT except the Special Administrative Regions of Hong Kong and Macau. The United States is RHT except the United States Virgin Islands.[13] The United Kingdom is LHT, but its overseas territories of Gibraltar and British Indian Ocean Territory are RHT.

Other Languages
العربية: جهة السير
čeština: Strana provozu
Esperanto: Trafikflanko
Frysk: Links ride
한국어: 대면 통행
Bahasa Indonesia: Arah lalu lintas
italiano: Mano da tenere
lumbaart: Man de tegnì
македонски: Страна на возење
Nederlands: Links rijden
日本語: 対面交通
srpskohrvatski / српскохрватски: Promet lijevom i desnom stranom
українська: Лівосторонній рух
粵語: 行車方向