Why do the British drive on the left side? This may be a question for you and here are some answers to you questions.
Up to the late 18th century, driving on the left was general in Europe. So why should all milestones and signs be put to the right? In the late 50ies people in Britain thought about changing to the right, like in Sweden. But they dimissed the thought, because of the costs (steering weels in cars, signs etc.). And Britain is an island, so there was no need to change to the right. And the British kept a little of their ›splendid isolation‹.
In Roman times the shield was carried with the left hand and the sword with the right. The soldiers marched on the left, so they could protect their body with their shield and they were able to fight with their right hand.
A horse is mounted from the left. You swing the right leg over the horse's back. To make it easier for smaller people to mount the horse, special stones (mounting stones) were provided. They were put on the left side of the roads.
Josef Mertens, Neuss and Anders Hanquist, Stockholm
Battles are fought via the left wing, like in soccer. Napoleon fought his battles via the right wing. It made him successful as his enemies didn't expect this strategy. Napoleon ordered that people had to drive on the right. In countries where Napoleon did not invade, people still drove on the left. The drivers of old stagecoaches sat on the right.
On 3rd September 1967, the change from the left to the right side took place in Sweden, due to practical reasons. This was the day H = höger. It means right in Swedish. Iceland followed in 1968.
Josef A. Winkelhofer, Azmoos (Switzerland)
Horses were harnessed one behind the other in England. The reins were drawn with the left hand, so you had to sit on the right. That's why the people drove on the right, in order to get a better view of the road.
I must point out that in days of old logic dictated that when people passed each other on the road they should be in the best possible position to use their sword to protect themselves. As most people are right handed they therefore keep to their left. This practice was formalised in a Papal Edict by Pope Benedict around 1300AD who told all his pilgrims to keep to the left.
Nothing much changed until 1773 when an increase in horse traffic forced the UK Government to introduce the General Highways Act of 1773 which contained a keep left recommendation. This became a law as part of the Highways Bill in 1835.
Jeremy Davis, Madrid (Spain)
I must point out that Napoleon was lefthanded, and so he used to draw his sword from right to left. He imposed his soldiers to parade marching on the right. Therefore, all Napoleon's conquests were changing the way carts and horses used to go. From left to right. The US after the War of Independence changed too, and so did Canada due to the French influence.
Commonwealth countries and other ones such as Japan, didn't change the way.