Why some countries drive on the left and others on the right
Any rule of the road works, as long as everyone follows the same one. Two carts meeting head-on miss each other if both pull to the same side. Which side a society chose is much less clear. Popular explanations involve swords, whips and the hand that held the reins, and a story that a pope told pilgrims in 1300 which way to keep. No documents support the pope story, and the true origin is not known.
Some evidence is old. Wear on the kerbstones of Pompeii suggests that where Roman carts had to share a street in two lanes, they kept to the right. In England, a London order of 1669 posted a man on London Bridge to keep carts going each way on their own side, and an Act of 1756 told traffic leaving London to use the bridge's east side: keeping left.
The United States went the other way. Its first keep-right law, in 1792, covered the Philadelphia and Lancaster Turnpike. A common explanation points to big freight wagons with no driver's seat, whose drivers rode the left rear horse and so wanted to pass oncoming wagons on their left. A popular story that Napoleon moved Europe to the right has no documentary evidence behind it. In 1827 a visitor to Paris wrote that its coachmen kept no fixed rule at all.
Empires and neighbours
Whatever the origins, empires spread the habits. Britain took keeping left to India, Malaysia, Hong Kong and much of East Africa. Every country that was part of the French colonial empire drives on the right. Japan, never part of the British Empire, keeps left by a custom from the Edo period, officially acknowledged in 1872 when its first railway was built with British help, and made law in 1924.
Then neighbours and cars pulled countries towards the right. In 1919 the world was evenly split, with 104 territories on each side. Between 1919 and 1986, 34 of the left-hand ones switched, for uniformity with neighbours, a change of government, or cheaper vehicles.
Sweden's switch is the best known. All its neighbours drove on the right, and more than 90 percent of Swedes already drove left-hand-drive cars, which made overtaking on narrow roads dangerous. Voters still said no in a 1955 referendum. Parliament decided on the change anyway, and on 3 September 1967, Dagen H, the whole country moved to the right in a single morning.
Today 165 countries and territories drive on the right and 75 on the left. The left-hand side covers about a sixth of the world's land, a quarter of its roads and about a third of its people.
Why did Sweden move to driving on the right in 1967?
Which side a country drives on owes less to logic than to history: whose empire it was in, who its neighbours are, and where its cars come from.
