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.
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 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.