Nadnerb is right... Point of view from a French, trying to write in English :
When we asked why the English drive on the left, we should rather wonder why we drive on the right ...
Napoleon - actually, the English saying that they are not like everyone else, but it is ultimately because of our good French old emperor often criticized that we (and the rest of Europe and the colonies) to roll right.
In the Middle Ages, it was de rigueur for right-handed knights to wear their swords left for chopping more quickly to fight against another knight. The left being only very few and to facilitate the movement on the roads, traffic horse naturally conducting the left to avoid the weapons of riders coming in one direction only affect those of other riders on small paths (which could be a sign of challenge to a duel).
To return to our sense of movement to the right, that was the strategist Napoleon thought it would be better to attack by the right to play the surprise (all others before him had often attacked by left). His technique that allowed repeatedly beat the Austrians (but never English), he decided to impose the flow direction right at all colonized countries in these major European battles to assert its superiority.
The direction of flow was then applied to all the colonies of countries invaded by Napoleon, unless of course the countries colonized by the British (only 61 of the 200 countries that have our planet).
Finally, note that a single street borrows from the right across the UK, it is a small street in London (Savoy Court) leading to the luxurious Savoy Hotel. This street is on the right because the drivers were accustomed to take in this regard to avoid having to go down to open the door to their passengers ...