Our practice of driving to the right can be traced back to Ancient Rome, where soldiers carried shields in their left hands. To protect themselves from strangers on the road, they walked on the right side so that the shield on the left provided that protection. As the Roman Empire spread over Europe, people followed that military custom of walking to the right.
By the year 1000, horse-drawn wagons began to fill the roads. These teams of horses were not driven from behind as we do now, but rather by men who led the teams on foot, walking in front of them. They led these horses with the right hand: therefore, the horses had to be on the driver’s right side.
When drivers started riding their teams, they usually sat on the left rear horse, giving themselves a free right hand for their whips.
When guns appeared in the 1400s, a rider on horseback usually carried his weapon in his left arm while riding on the right. In this position, his gun was always pointed at a passing stranger.
This pattern, however, underwent a change in England, a change brought about by knights who enjoyed the sport of jousting on horseback. They charged at each other, in full armor, carrying swords and spears, and tried to knock each other off the horses. To do this, the knight had to ride on the left and pass his opponent on his right, the side on which he was holding his weapon.
The English people soon began to imitate the knights, and traffic in that country stayed to the left, passing on the right, as they still do today.