Ancient Greek, Egyptian, and Roman troops kept to the left when marching. 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.
The first reference in English law to an order for LHT was in 1756, with regard to London Bridge.
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. In the year 1300, Pope Boniface VIII directed pilgrims to keep left.
The handling etiquette for Japanese swords, particularly the katana, partly involved the avoidance of what was once called saya-atte, or scabbard-striking, obligating the persons involved to immediately engage in combat - to avoid such incidents, samurai walking or riding on horseback by each other on a road were obligated to follow LHT-form traffic rules. It is also thought that British falconers on hunts always had their raptors perched on their leather gauntleted left arm, to keep their right hand free for longsword use, and similarly needed to follow LHT-form traffic rules for many of the same reasons as samurai did.
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.
In France, traditionally foot traffic had kept right, while carriage traffic kept left. Following the French Revolution, all traffic kept right. Following the Napoleonic Wars, the French imposed RHT on parts of Europe. 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. New York formalized RHT in 1804, New Jersey in 1813 and Massachusetts in 1821.
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 China of Hong Kong and Macau. The United States is RHT except the United States Virgin Islands. The United Kingdom is LHT, but its overseas territories of Gibraltar and British Indian Ocean Territory are RHT.
Influential in Europe was the 1920 Paris Convention, which advised driving on the right-hand side of the road, in order to harmonise traffic across a continent with many borders. This was despite the fact that left-hand traffic was still widespread: in 1915 for example, LHT was introduced everywhere in the Austro-Hungarian Empire. However, three years later the Empire was split up into several countries, and they all changed eventually to RHT, notably including when Nazi Germany introduced RHT with almost immediate effect in Czechoslovakia in 1938-39.
Sweden was LHT from about 1734 to 1967, despite having land borders with RHT countries, and approximately 90 percent of cars being left-hand drive (LHD) vehicles. A referendum was held in 1955, with an overwhelming majority voting against a change to RHT. Nevertheless, some years later the government ordered a conversion, which took place at 5 am on Sunday, 3 September 1967. The accident rate dropped sharply after the change, but soon rose back to near its original level. The day was known as Dagen H ("H-Day"), the 'H' being for Högertrafik ("right traffic"). When Iceland switched the following year, it was known as H-dagurinn, again meaning "H-Day".
In the late 1960s, the UK Department for Transport considered switching to RHT, but declared it unsafe and too costly for such a built-up nation. Road building standards, for motorways in particular, allow asymmetrically designed road junctions, where merge and diverge lanes differ in length.
Today, four countries in Europe continue to use left-hand traffic, all island nations: the UK, Cyprus, Ireland, and Malta. The unrecognized Turkish Republic of Northern Cyprus continued to drive on the left after separating from the south in 1974.
Asia and the Pacific
Nationalist China adopted RHT in 1946. This convention was preserved when the CCP took the mainland and the KMT retreated to Taiwan. Hong Kong and Macau continue to be LHT.
Both North Korea and South Korea switched to RHT in September 1945 after liberation from Japan which was defeated and surrendered by the Allies.
Myanmar switched to RHT in 1970.
Samoa, a former German colony, had been RHT for more than a century. It switched to LHT in 2009, being the first territory in almost 30 years to switch. The move was legislated in 2008 to allow Samoans to use cheaper right-hand drive (RHD) vehicles--which are better suited for left-hand traffic--imported from Australia, New Zealand or Japan, and to harmonise with other South Pacific nations. A political party, The People's Party, was formed to try to protest against the change, a protest group which launched a legal challenge, and an estimated 18,000 people attending demonstrations against it. The motor industry was also opposed, as 14,000 of Samoa's 18,000 vehicles are designed for RHT and the government has refused to meet the cost of conversion. After months of preparation, the switch from right to left happened in an atmosphere of national celebration. There were no reported incidents. At 05:50 local time, Monday 7 September, a radio announcement halted traffic, and an announcement at 6:00 ordered traffic to switch to LHT. The change coincided with more restrictive enforcement of speeding and seat-belt laws. That day and the following day were declared public holidays, to reduce traffic. The change included a three-day ban on alcohol sales, while police mounted dozens of checkpoints, warning drivers to drive slowly.
The Philippines was mostly LHT during its Spanish and American colonial periods, as well as during the Commonwealth era. During the Japanese occupation the Philippines remained LHT, also because LHT had been required by the Japanese; but during the Battle of Manila the liberating American forces drove their tanks to the right for easier facilitation of movement. RHT was formally finalised by Executive Order No. 34 signed by President Sergio Osmeña on 10 March 1945.
A number of non-contiguous former British colonies in West Africa originally drove LHT and switched to RHT in the early 1970s to match the surrounding countries. Sierra Leone switched to RHT in 1971, Nigeria in 1972 and Ghana in 1974. Before this period The Gambia, a country entirely contained within RHT Senegal, had officially switched to RHT in 1965.
Rwanda, a former Belgian colony in central Africa, is RHT but is considering switching to LHT, to bring the country in line with other members of the (EAC). A survey, carried out in 2009, indicated that 54% of Rwandans were in favour of the switch. Reasons cited were the perceived lower costs of RHD vehicles as opposed to LHD versions of the same model, easier maintenance and the political benefit of harmonisation of traffic regulations with other EAC countries. The same survey also indicated that RHD cars are 16 to 49 per cent cheaper than their LHD equivalents. In 2014 an internal report from consultants to the Ministry of Infrastructure recommended a switch to LHT. In 2015, the ban on RHD vehicles was lifted; RHD trucks from neighbouring countries cost $1000 less than LHD models imported from Europe.