Driving on the left is not the deviation. It is the original arrangement, and most of the world switched away from it.
Roughly a third of the world's population still drives on the left, across about 75 countries and territories. The split is not random: it traces the reach of the British Empire almost exactly, with a few instructive exceptions.
Why left came first
The traditional explanation involves swords, and it holds up reasonably well.
Most people are right-handed. A right-handed rider mounts a horse from the left, which is easier at the roadside than in the middle of the road. A right-handed rider also wants their sword arm towards oncoming strangers. Both point to keeping left.
There is archaeological support. A Roman quarry track in Britain shows deeper wear on the left-hand side going in, consistent with loaded carts keeping left. Pope Boniface VIII is recorded advising pilgrims to keep left in 1300.
Why so much of the world switched
The change came from freight, not from riders.
Large American wagons in the eighteenth century were pulled by teams of horses and had no driver's seat. The driver rode the rear left horse, keeping the whip hand free. Sitting on the left, the driver wants oncoming traffic passing on their left so they can judge the gap, which means driving on the right.
Then two accelerants. Revolutionary and Napoleonic France standardised on the right and exported it across the territories it occupied, which is most of continental Europe. And Henry Ford put the steering wheel on the left of the Model T in 1908 and made twelve million of them, which settled the question for every country buying American cars.
Where the line falls today
| Left-hand traffic | Right-hand traffic |
|---|---|
| United Kingdom and Ireland | Continental Europe |
| India, Pakistan, Bangladesh | China, Russia, most of Asia |
| Japan, Thailand, Indonesia | The Americas, except a few islands |
| Australia, New Zealand | Most of Africa |
| Much of southern and eastern Africa | The Middle East |
Japan and Thailand are the interesting entries, since neither was British-ruled. Japan's convention predates the cars and was reinforced when British engineers built its railways in the 1870s. Thailand simply kept left and never changed, though it is now surrounded by right-hand-traffic neighbours.
The countries that did switch
Sweden is the famous case. On 3 September 1967, at 5am, the country changed from left to right in a single coordinated moment known as Dagen H. Traffic stopped, vehicles moved across, and traffic restarted.
It was planned for years: every intersection redesigned, every sign replaced overnight, buses rebuilt with doors on the other side. Accident rates fell immediately afterwards, because everyone was driving with extreme care, then returned to trend within about eighteen months.
Notably, Sweden switched despite a referendum in which 83 percent voted against. The government did it anyway, on the argument that neighbouring Norway and Finland drove on the right and most Swedish cars were already left-hand drive.
What it costs to be the odd one out
Very little, most of the time, which is why the split persists.
Cars are manufactured in both configurations. Drivers adapt within a day or two. The only genuinely awkward cases are land borders between countries of opposite convention, and those are handled with a small number of purpose-built crossovers: bridges at the Thai border, and a well-known one at the Hong Kong and mainland China boundary.
Switching, by contrast, is expensive and dangerous, and the benefit is limited unless you share a busy land border. The calculation almost always says stay.
Nobody is on the wrong side. Both conventions work perfectly well, and the only thing that matters is that everyone in a given place agrees on which one it is.
Why it never resolves
This is a pure coordination problem, and coordination problems do not resolve on their own. Everyone's best move depends entirely on what everyone else does, and no participant gains by changing alone.
The same shape explains why the seven-day week survived two determined attempts to replace it, and why tipping persists in places where nearly everyone involved says they dislike it. Once the whole system is coordinated on an arrangement, being right is worth less than being aligned.
It is also a good corrective to the idea that everything is converging. Global supply chains have made a lot of the world look alike, right down to the interior of the nearest café. Which side of the road you drive on has resisted all of it, because the cost of changing falls on the country that changes and the benefit is shared.
The short answer
Left was the default until wagons, Napoleon and Ford pushed most of the world to the right. The countries that kept left mostly did so because Britain was there when the rules were written, and none of them has switched since because the arrangement works and changing it does not pay.