Throughout the world, countries observe either left-hand traffic (LHT) or right-hand traffic (RHT) guidelines, which determine which side of the road a driver will stay on when traveling. In LHT areas, drivers stay on the left side of the road (except under special circumstances such as passing another vehicle), while RHT areas require drivers to stay on the right side of the road whenever possible. Roughly 30% of the world's countries and territories are LHT nations, comprising about 35% of the global population and roughly one-quarter of the world's roadways. By comparison, the 160+ countries and territories that drive on the right and hew to RHT guidelines include 65% of the population and 75% of the roadways.
Most LHT (also known as "keep-left") countries and territories are former British colonies, many of which remain part of the British Commonwealth. In the early 20th century, approximately half of the world's countries (104) drove on the left side of the road. However, 34 of those countries transitioned to RHT ("keep-right") systems between 1919 and 1986, resulting in the current balance:
Countries and Territories that Drive on the Left:
| Akrotiti and Dhekelia | Fiji | Mauritius | Seychelles |
| Anguilla | Grenada | Montserrat | Singapore |
| Antigua and Barbuda | Guernsey | Mozambique | Solomon Islands |
| Australia | Guyana | Namibia | South Africa |
| Bahamas | Hong Kong | Nauru | South Georgia & South Sandwich Islands |
| Bangladesh | India | Nepal | Sri Lanka |
| Barbados | Indonesia | New Zealand | Suriname |
| Bermuda | Ireland | Niue | Tanzania |
| Bhutan | Isle of Man | Norfolk Island | Thailand |
| Botswana | Jamaica | Northern Cyprus | Timor-Leste |
| British Virgin Islands | Japan | Northern Ireland | Tonga |
| Brunei | Jersey | Pakistan | Trinidad and Tobago |
| Cayman Islands | Kenya | Papua New Guinea | Turks and Caicos |
| Christmas Island | Kiribati | Pitcairn Islands | Tuvalu |
| Cocos (Keeling Islands) | Lesotho | Saint Helena, Ascension and Tristan da Cunha | Uganda |
| Cook Islands | Macau | Saint Kitts and Nevis | United Kingdom |
| Cyprus | Malawi | Saint Lucia | United States Virgin Islands |
| Dominica | Malaysia | Saint Vincent and the Grenadines | Zambia |
| Eswatini | Maldives | Samoa | Zimbabwe |
| Falkland Islands | Malta |
Left-hand traffic and right-hand traffic are related to—and easily confused with—the terms left-hand drive and right-hand drive, which indicate which side of the actual vehicle the steering wheel is located. Because the driver is meant to sit on the side closest to the centerline of the road, right-hand-drive (RHD) vehicles are standard in left-hand-traffic (LHT) countries. By the same token, left-hand-drive (LHD) vehicles are standard in right-hand-traffic (RHT) countries.
These standards are rarely legal requirements. In most countries, the law requires LHT or RHT but drivers are free to own, register, and drive both RHD and LHD vehicles. In fact, many specialized vehicles, such as mail delivery trucks or trash collection trucks, are deliberately designed to parallel the traffic type (RHD for RHT and LHD for LHT) to give the driver quicker access to the curb or sidewalk. However, a few nations, such as New Zealand and Singapore, forbid the use of same-side drive (also called "wrong-hand-drive") vehicles.
Perhaps surprisingly, many countries enshrined LHT or RHT into their traffic laws long before automobiles came to dominate the roadways. For example, RHT has been standard in the United States since colonial times, when most vehicles on the roads were horse-drawn wagons. RHT was coded into the laws of various states and roadways beginning in 1792—more than 100 years before pioneering businessmen such as Ransom Olds and Henry Ford would use mass-production techniques to make automobiles affordable to the masses. Numerous theories exist as to why the U.S. chose RHT. These theories range from the fact that most wagon drivers were right-handed (and so held the reins with their left hand and a whip or weapon in their right) to the possibility that the early colonists just wanted to be as different from mother England as possible.