I noticed a this topic on a forum recently and thought it would be a good thing publish some sensible stuff about it.
No one is absolutely and definitely certain why the UK drives on the left and everyone else (about three quarters of the world) on the right. However, there are some very reasonable arguments about why this is (I got this from Tesco Bank). It’s worth noting that in order to understand these arguments, you have to consider an extended historical timeline. You can’t just take one piece of historical information and then poo-poo it in a modern context. Well, I say that – don’t forget that we’re talking about driving instructors here, and they can do things like that at the drop of a hat!
In the past, when swords were the weapon of choice, most people were right-handed. It was therefore common practice to walk on the left and pass other travellers sword arm to sword arm. Many years later, in the mid-18th century, the General Highways Act was introduced and this practice was carried over with the recommendation that traffic keep to the left. This was then further carried on into the Highways Bill in the early part of the 19th century.
In France, there was probably a similar approach to start with, but when Napoleon came to power, the fact that he was left-handed meant that he marched on the right. So France adopted this, and since both France and Britain were active colonisers any country they colonised was forced to adopt the respective system.
America was initially colonised by Britain, France, Spain, and Portugal. Britain was a minority player, and so using the left-hand side of the road was never adopted over there. As America grew, many other countries changed their system to match.
It would cost billions for the UK to change now.
Like it or not, swords are part of the reason why we drive on the left. There’s no single reason why the world does what it does, but an interconnected series of historical situations that stretch back to mediaeval times.