Why do some countries drive on the left?

Roughly 65 countries drive on the left, accounting for about a third of the world's population, and the overwhelming majority are former British colonies or territories including India, Australia, Japan, South Africa, and much of the Caribbean and Southeast Asia. Japan is a notable exception to the colonial pattern, having adopted left-hand traffic independently through custom and later British influence on its railway construction.

The traditional explanation for left-hand travel predates automobiles. Most people are right-handed, and a rider passing a stranger on the left keeps his sword arm between them, while also making it easier to mount a horse from the left side away from traffic. Whether that is the actual historical cause or a plausible retrofit is debated, but left-hand travel was common in medieval Europe and was formalized in Britain by law in the 18th and 19th centuries.

The shift to the right spread through different mechanics. Large wagons in France and later the United States were driven by a postilion riding the left rear horse, which gave better visibility when passing on the right, and revolutionary France standardized right-hand traffic, spreading it across territory under Napoleonic influence. American practice reinforced it globally through automobile manufacturing. Several countries have switched, and Sweden's change in 1967, known as Dagen H, was executed nationally on a single September morning with extensive preparation and, contrary to expectation, produced a temporary decrease in accidents as drivers concentrated. Switching is now considered impractical for most countries given road infrastructure and vehicle fleets, which is why the split persists and why driving abroad requires attention to more than the side of the road, including which side roundabouts turn and where the vehicle's controls sit.