Right- and left-hand traffic

The terms right-hand traffic (RHT) and left-hand traffic (LHT) refer to regulations requiring all bidirectional traffic, unless otherwise directed, to keep to the right or to the left side of the road, respectively. This is so fundamental to traffic flow that it is sometimes referred to as the rule of the road.

About two thirds of the world's population (163 countries and territories) are RHT, with the remaining 76 countries and territories LHT. Countries that use LHT account for about a sixth of the world's area and a quarter of its roads. In the early 1900s some countries like 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. 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 sides.

Most regions with concentrations of LHT are where there were once many British colonies such as the Caribbean, southern Africa, Southeast Asia, Australia, and New Zealand. In Europe, only four countries still drive on the left: the United Kingdom, Ireland, Malta, and Cyprus, all of which are islands. Japan, Thailand, Nepal, Bhutan, Mozambique, Suriname, East Timor and Indonesia are among the few LHT countries outside the former British Empire.

Nearly all countries use one side or the other throughout the entire country. Exceptions are due to historical considerations and involve islands not attached the main 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.

...
Wikipedia