Why do some countries drive on the left?
Left was the older custom across much of the world, and the countries that changed did so because of specific historical pressures — mainly Napoleon, and later the pull of neighbours and American vehicles.
Simple intuition
The plain reason, in everyday words
There is no advantage to either side. What matters overwhelmingly is that everyone in one place agrees, because the cost of disagreement is head-on collisions. So the question is not which side is better but how each country's answer got locked in. The usual account is that keeping left was the older habit in much of Europe: a right-handed rider passing a stranger on the left keeps his sword arm between them, and mounting a horse from the left is easier if you carry a sword on your left hip. Then France changed, Napoleon exported the change across the territories he controlled, and the map began to split. After that, each country's roads, vehicles and habits accumulated around whichever answer it had, and switching became progressively more expensive until almost nobody did.
One side is safer than the other.
Comparisons between left- and right-driving countries find no robust safety difference attributable to side. What matters is that everyone in a country agrees and that the vehicles match the convention.
Countries drive on the left because they were British colonies.
Colonial administration spread the convention widely, but Japan adopted it independently, and several former British territories switched later under pressure from neighbours. The correlation is strong but not the whole explanation.
Switching sides is impossible.
Sweden did it in 1967 and Samoa in 2009. It is expensive and requires enormous preparation, but the obstacle is cost and coordination, not feasibility.
The medieval sword-arm explanation is established fact.
It is a plausible and widely repeated story with limited direct evidence. Practice appears to have varied regionally, and the wagon-driving argument for right-side travel is better supported.
It is the most tangible everyday example of path dependence: an arbitrary early choice, locked in by everything constructed on top of it, and now unchangeable at any sensible price. The same structure explains keyboard layouts, rail gauges, mains voltages, character encodings and file formats — all cases where the world would choose differently starting from scratch and cannot get there from here.
Who worked it out
Keeping left appears to have been common in medieval Europe, with the usual explanation being that a right-handed traveller keeps his weapon hand toward oncoming strangers, though the direct evidence is limited.
What problem forced it
France moved to the right around the Revolution, and Napoleon's conquests spread the practice across much of continental Europe, splitting the map along political lines rather than practical ones.
How it changed since
The British Empire exported left-hand travel worldwide, mass-produced American vehicles pushed others toward the right, and a handful of countries have switched since — Sweden in 1967, Iceland in 1968, Samoa in 2009 — each for specific practical reasons rather than principle.
Why QWERTY survived
The same lock-in argument, with a much better-documented origin and a long history of failed replacements.
How rail gauges got standardised
Another arbitrary choice frozen by infrastructure, with the added complication of networks meeting at borders.
Written for Curio rather than collected from a forum — it is part of the curated corpus that ships with the platform. The references it draws on are listed under Sources.
Why do keyboards use QWERTY?
It was designed around the mechanical constraints of a machine nobody uses any more, and it survived because everyone had already learned it.
A natural next question