The most fundamental traffic law in any country is whether traffic will follow the principles of right-hand traffic (RHT, or "keep right"), in which vehicles travel predominantly on the right side of the road, or left-hand traffic (LHT, or "keep left"), in which vehicles keep largely to the left.
Left-hand traffic rules are followed by roughly 30% of the world's countries and territories and 35% of the global population, including the United Kingdom and many former British colonies. However, the majority of the world's UN-recognized countries, which include 65% of its population and 75% of its roadways, follow right-hand traffic rules.
Countries and Territories that Drive on the Right:
Right-side traffic vs right-side drive
The terms right-hand traffic (RHT) and left-hand traffic (LHT) are easily confused with the related terms right-hand drive (RHD) and left-hand drive (LHD). While right-hand traffic indicates the side of the road traffic travels on, right-hand drive refers to the side of the vehicle on which the steering wheel and gas/brake pedals are located.
Early cars often placed the steering wheel either in the center of the vehicle or on the driving side. This changed with the 1908 Model T Ford, which moved the steering wheel closer to the center of the road to give the driver a better view of oncoming traffic. Other manufacturers quickly followed suit. As such, right-hand traffic countries are dominated by left-hand drive vehicles, while left-hand traffic countries prefer right-hand-drive vehicles.
Despite these trends, vehicles of both right-hand drive and left-hand drive are legal in most countries. Only a few countries, such as Armenia, Chile, Saudi Arabia, Sierra Leone, and Taiwan forbid the registration and use of same-side drive (also called "wrong-hand-drive") vehicles such as RHD vehicles in a RHT country. In fact, mail trucks, sidewalk sweepers, and other vehicles that benefit from quick access to the sidewalk are often deliberately manufactured with same-side drive.
Ancient traffic laws and the shifting balance of left-side driving
In most countries, traffic laws originated in the era of the horse-drawn wagon, centuries before cars and other motor vehicles became widely available. For example, various states in the US began legally establishing right-hand traffic laws as early as 1792, more than 100 years before mass-produced automobiles arrived on the market.
Historians are split as to why the US chose right-hand traffic. One prevailing theory is that RHT felt more natural for wagon drivers—most were right-handed, so they preferred to hold the reins with their left hand and keep their right hand free to wield a rifle or whip. However, another popular theory holds that the fledgling country simply wanted to do the opposite of England, which followed LHT guidelines.
In the early 1900s, the number of nations and territories that drove on the right (RHT, or "keep right" countries) was roughly equal to the number of nations and territories that drove on the left (LHT, or "keep left" countries). However, from 1919 to 1986, more than 30 countries, many of which were former British colonies and remained part of the British Commonwealth, switched to RHT.