In ancient Rome, according to archaeologists, the rule of "keep left" was first established. Examining the road construction of that era, they reached to the conclusion that the vehicles and animals moved to the left side of the road.
This habit seems to be continued in the Middle Ages, too. The riders moved on the left side of the road, to have their right hand free, in case they had to draw their sword to defend themselves against the enemy, which might have been coming from across the street. This habit became an institution in 1300 by decision of the Pope Boniface the 8th.
The system was changed in America by a law passed in 1792 in Pennsylvania, in order to better manage the number of vehicles, which was constantly increasing. In Europe, the change came due to the French Revolution of 1789. Until then, the carriages of the aristocracy were exclusively circulating on the left side of the road, while the poor were on the opposite side. After the revolution, the nobility and the rich began to drive on the right, trying not to be distinguished, in order to escape the fury of the people.
England never changed the rule, something that may be associated to the country’s long term rivalry with France. In 1835 the rule of "keep left" was established by law. Consequently, more than 50 countries and dozens more British territories in countries, which in their majority are former colonies of the Great Britain, are now still following the rule of driving on the left. The rule also applies in several “non-English” countries, such as Japan.
This means that cars drive on the left and they choose the right side to pass by a vehicle that is moving ahead. In roundabouts the traffic is clockwise. So those drivers entering the roundabout must give way to the traffic coming from the right.
Countries that drive on the left are marked with blue.
