Why do countries drive on different sides of the road?

There is no single cause. For centuries the side of the road was a local custom, shaped by how people travelled and by the fact that most people are right-handed. Later, national laws, colonial empires and car design fixed the habits in place, and the cost of changing them keeps most countries where they are. One researcher quoted by Autoweek concludes that what is not known about the subject still outweighs what is known. [1] [2]

Writers commonly say that about 90 to 94 percent of people are right-handed, and that this shaped early rules. One account holds that armed riders and walkers favoured the left, so that the right hand, the stronger one, was toward any stranger they passed. Autoweek's account applies this to samurai-era Japan and knight-era England. History.com says archaeological evidence suggests the ancient Romans may have driven on the left, though it calls the reasons uncertain. [2] [3] [1]

National Geographic gives a different version for the Romans: charioteers held the reins in the right hand and whipped with the left, so they would keep to the right to avoid whipping passing chariots. The sources therefore disagree on which side the Romans used and why. The weapon theory is also described only as something some people believe, and the sources give no firm evidence for it. [4] [3]

Until the 1700s, traffic was light and the side often varied locally. Large freight wagons pulled by teams of horses then pushed some regions to the right. Drivers often rode the left rear horse so that the right hand could control the team, and they kept right to judge clearance on their left. The Federal Highway Administration's historian Albert Rose, and the Australian historian M. G. Lay, saw the Conestoga wagon of about 1750 as a major influence in America. Pennsylvania wrote a keep-right rule into a turnpike charter in 1792, and New York was the first state to require it on all public highways, in 1804. [5] [3]

Britain made left-hand traffic law through measures in 1773 and 1835, while France favoured the right as early as the 18th century. Each exported its rule to its colonies, which is why Australia, New Zealand, South Africa and India drive on the left. Many African states later changed to match their neighbours. [3] [4]

Early cars often had central tillers. After the steering wheel arrived in 1898, makers usually put the driver on the curb side, so many American cars before 1910 had right-side seating. The 1908 Model T was Ford's first car with a left-side driver position, and Ford is cited as a major influence on the right-hand rule in the United States. [5] [4]

Several countries switched to the right: Canada, Italy and Spain in the 1920s, much of Eastern Europe in the 1930s, and Sweden in 1967, at a cost of about $120 million in preparation. Sweden's case was driven by left-hand-drive cars, risky overtaking, and border collisions with neighbours who drove on the right. Close to 50 countries still drive on the left. National Geographic attributes this mainly to momentum: road systems built around one side are expensive and complex to change. [4] [3] [6]