75 countries drive on the left. 165 drive on the right. That is about a sixth of the
world's land, a quarter of its roads and roughly a third of its people — and almost every
popular explanation for it turns out to be undocumented.

You have probably heard that knights kept left so their sword hand stayed free, or that
Napoleon flipped Europe to the right. Historians have gone looking for evidence of both
and found none. What does exist is a law: the London Bridge Act 1756 required carriages
leaving London to keep to the east side — the left by direction of travel — which may be
the first time left-hand traffic was written into statute anywhere.

Britain wrote it down and then built an empire, and wherever British engineers laid the
roads and the railways the left went with them: India, Pakistan, Bangladesh, Australia,
New Zealand, South Africa, Kenya, Malaysia. The left-hand bloc is very close to a map of
the British Empire. The big exception is Japan, which was never a British colony but built
its first railway in 1872 with British engineering help; the trains ran on the left and
the roads followed.

North America went the other way, and that story does have evidence behind it. American
freight wagons had no driver's seat — the teamster rode the rear-left horse, so he sat on
the left, and a driver sitting on the left wants oncoming traffic passing on his left.
That means keeping right.

Then there are the countries that changed their minds. Sweden drove on the left until
05:00 on 3 September 1967, when every vehicle in the country stopped, moved across and
waited: Dagen H, at a cost of about £40 million at the time. Accidents dropped sharply,
then drifted back to normal. Iceland followed the next May; Okinawa went the other way in
1978. In 2009 Samoa switched from right to left so it could import cheaper used
right-hand-drive cars from Australia, New Zealand and Japan, declaring a two-day holiday
and a three-day alcohol ban to get through it.

And where the two systems meet, someone has to build a swap. On Thailand's borders with
Laos, Cambodia and Myanmar, traffic changes sides at a set of lights. The Takutu River
Bridge between Guyana and Brazil is the only crossing in the Americas that does it. And
Myanmar switched to the right in 1970 but still imports right-hand-drive cars from Japan,
so most drivers there sit on the kerb side of their own car.