The reason that drivers in the U.K. drive on the left side of the road goes back to the days when people rode horses and carried swords. In those times, it was much easier for a right-handed rider (as would have been more common) to fend off potential attackers coming from the opposite direction whilst staying to the left side of a street. This tradition remained in place despite the advent of driving horses and carts on the right, which spread throughout continental Europe, particularly under the influence of Napoleon; it was most likely a reflection of the tendency of teamsters driving a pair of horses to sit on the horse on the left, so that the driver could control the horses with his/her right hand.

In the early days of the American colonies, riders were expected to adhere to British law; however, upon gaining independence from England, and in no small part thanks to the influence of immigrants from countries where driving on the right side of the road was the norm (the law requiring Russians to drive on the right was passed in 1752; the French law was passed in 1794) they switched from the left to the right. In Canada, however, which was still occupied by the English, drivers in some provinces drove on the left until as late as 1947.

Sources : 1, 2