It seems that the British driving pattern derives from early days, days of horses and horsemen. Evidently, it seemed only reasonable that a horseman would meet another horseman by keeping to the left. This would enable the horseman to remain in the most advantageous position in the event that he would need to draw his sword quickly to defend himself against the on-coming horseman. Given that most people are right handed, approaching another on the left would enable the horseman to employ his right hand more freely. According to one report that I have read, Pope Benefice issued a Papal Edict around A.D. 1300 requiring all to keep to the left on roadways.
In Britain under King George III the Government issued the General Turnpike Act of 1773. Of its various provisions, one seems to have been commendation of keeping to the left on public roadways and streets.
If meeting oncoming horsemen on the left was advantageous and stuck in Britain, why did other countries decide to have traffic meet oncoming vehicles on the left? One offers the following explanation, at least for France.
Reasons to travel on the right are less clear but the generally accepted version of history is as follows: The French, being Catholics, followed Pope Boneface's edict but in the build up to the French Revolution in 1790 the French Aristocracy drove their carriages at great speed on the left hand side of the road, forcing the peasantry over to the right side for their own safety. Come the Revolution, instincts of self preservation resulted in the remains of the Aristocracy joining the peasants on the right hand side of the road. The first official record of this was a keep right rule introduced in Paris in 1794.
Political events in France had a big effect on driving habits. Before the Revolution of 1789, the aristocracy drove its carriages along the left side of the roads, forcing the peasants to the other side. But once the Revolution started, these nobles desperately tried to hide their identity by joining the peasant travelers on the right. By 1794 the French government had introduced a keep-right rule in Paris, which later spread to other regions as the conquering armies of Napoléon I marched through much of continental Europe. It is not surprising that Napoléon favored keeping to the right. One reference work explains that because he was left-handed, “his armies had to march on the right so he could keep his sword arm between him and any opponent.”
In the late 1700’s, a shift from left to right took place in countries such as the United States, when teamsters started using large freight wagons pulled by several pairs of horses. The wagons had no driver’s seat, so the driver sat on the left rear horse and held his whip in his right hand. Seated on the left, the driver naturally preferred that other wagons pass him on the left so that he could be sure to keep clear of the wheels of oncoming wagons. He did that by driving on the right side of the road.