Archaeological evidence suggests that the ancient Romans may have driven their carts and chariots on the left, and the practice seems to have carried over into parts of medieval Europe. The reasons for this are not entirely certain, but some believe it arose as a matter of safety. The majority of people are right handed, one theory goes, so driving or riding on the left would have allowed them to wield a weapon with their dominant hand if they crossed paths with an enemy.

Until as recently as the 1700s, horse and wagon traffic was so light that the decision to drive on the left or right often varied according to local custom. Left-hand traffic finally became the law of the land in Britain after the passage of government measures in 1773 and 1835, but the opposite tradition prevailed in France, which favored the right as early as the 18th century. These two countries later exported their driving styles to their respective colonies, which is why many former British territories such as Australia, New Zealand, South Africa and India still drive on the left.

In the United States, meanwhile, many researchers trace the beginning of right-hand traffic to the 18th century and the rise of freight wagons pulled by large teams of horses. Since these vehicles often didn’t have a driver’s seat, drivers tended to ride on the left rear horse to more easily control their animal team with their right hand. As the wagons became more popular, traffic naturally moved to the right so drivers could sit closer to the center of the road and avoid collisions with one another. Yet another major influence was carmaker Henry Ford, who mass-produced his Model T with a left-positioned steering wheel, which necessitated driving on the right side of the road.

These days, left-hand traffic remains the norm in Britain and many of its former colonies as well as in Japan, Indonesia, Thailand and several other nations. Nevertheless, with the rise of the automobile, many countries have switched to the right to fit in with their neighbors. Canada abandoned the left side of the road in the 1920s to facilitate traffic to and from the United States. In 1967, meanwhile, the government of Sweden spent around $120 million preparing its citizens to begin driving on the right.