Throughout history most countries have driven on the left side of the road in accordance with left-side laws kept by each country. In fact, there is evidence of left-side driving as far back as the Roman Empire. The left-side driving rules even extended deep into the Middle Ages in Europe where drivers kept to the left so that they could easily reach for their weapons or extend a friendly hand to any passerby on the other side as needed. So how did some countries begin driving and travelling by the right side of the road?
During the 1700s, in the United States and France, drivers began using the right side of the road for all travel whether by horseback, carts, or any other means. At this time there were no laws for driving right, but both teams were possibly forced to switch driving sides and become right-based due to large wagons that were driven at the time. These wagons were hitched to teams of horses and the driver sat on the back left-hand horse. Some people believe it was easier for drivers to steer their horses if they kept them to the right, while others say that due to the large size of the horse teams and wagons it was easier to see whether or not you were too close to others if they drove on your left side and you stayed on the right.
By the 1800s right-side driving was passed as a law in the United States and up through Canada. At the same time, although France had once again started adopting left-side driving, Napoleon quickly declared laws for right-side driving and spread them in the countries he conquered such as Germany, Italy, and Spain. Countries that were not conquered such as England, Russia, and Portugal chose to stay left-side driving.
After World War I even more countries began making the switch to becoming right-side driving, but countries such as England and India have largely resisted. This is why today most countries throughout the world drive on their right-sides, but there are still some countries that drive on their left-side.