Well, as we all know, it’s the other way round in Britain.
If we appeal to studies we can see that there are several points of views why the UK is a jurisdiction with left-hand traffic. Some historians think that the origins of left-handed traffic come from the ancient times (not far from Swindon, England archaeologists has found the evidence that when the Romans drove their loaded vehicles from the quarry more deep dints were made on the left side of the road). The other version is that the medieval travellers chose the left side simply because of keeping the right hand free either for swords (!) in case horsemen might need to defend themselves or to salute the passing riders offering them friendship – as most people were right-handed – meant to passing each other on the left. Another opinion is that the left-hand traffic was adopted from the marine rules where a ship has to pass another ship approaching from the right side. However, the first time when left-hand drive rule became mandatory was in 18th century. That rule was introduced in 1722 by Lord Mayor, who ordered that London Bridge traffic (the place where London started from) had to be kept to the left in order to regulate intensive traffic on the narrow bridge – the first ever road rule in Britain.
Back then lots of countries stipulated keep-left rule, and the shift from left to right started mostly in 18th century in Continental Europe. Usually, the Continental way of driving on the right is associated with Napoleon. There is a story that keep-right rule was introduced by Napoleon because he was left-handed. However, the general opinion is that the rule to keep to the right was formed when Bonaparte ordered that the troops had to stay on the right side of the road thus every passer-by had to give the military the way.
Nowadays, the right-side rule is obviously prevailed, leaving only only 76 countries, territories and dependencies with the left-kept traffic.