The majority of todays nations drive on the right side of the road, following the rules of right-hand traffic, or RHT, including the United States, Canada, Russia, and much of Europe. But around one-third of the worlds countries buck the trend with left-hand traffic rules, or LHT. The United Kingdom is one of these nations, along with many of its former colonies, like Australia, and many Asian countries. The British practice has a long tradition, perhaps dating back to Roman Britain. So why do Brits drive on the left, and why do other countries drive on the right?Roman Roads Spread Roman CustomsEmperor Otho on Horseback, by Antonio Tempesta, 1596. Source: Metropolitan Museum of ArtOne of the most widely circulated theories about why the British drive on the left dates back to ancient Rome. Back then, travelers riding on horseback faced the constant threat of attack or mugging. Because most people are right-handed, riding horses on the left meant their right, dominant hand could be ready to draw a sword if necessary. History suggests that Roman soldiers maintained the tradition when marching on the many roads they constructed across the Roman Empire, spreading the custom with them.The Salt Lake Cutoff, Charles Carter photo. Source: True West MagazineDriving carts and other vehicles on the left was widespread around much of the former Roman Empire for centuries. However, in 1792, the first keep right law was passed in Pennsylvania, and many states across the US and Canada followed suit. In France, meanwhile, Napoleon enforced the right-hand driving rule across all French territories, influencing customs across mainland Europe.Keep Left Became British LawPope Boniface VIII, c. 1294-1303.The first officially sanctioned rule for driving on a specific side of the road was passed by Pope Boniface VIII in 1300 CE. This was declared a jubilee year, so they were expecting many pilgrims to head to Rome, whom they wanted to organize to collect taxes. As part of his efforts to control pilgrim movement, he declared that they should all travel on either the left or the right, as there is no documented evidence. Considering the Roman practice, the left seems likely.In 1773, as British roads became more developed and congested, the government brought in the General Highways Act, which stated that all horse riders, farmers, coachmen, and lawless highwaymen must remain on the left side to avoid any nasty collisions, even before cars had been invented. Just over a century later, the 1883 Highway Act made it a crime to drive on the wrong side of the road.British Cars Were Designed with Drivers on the RightThe interior of a classic British car with the drivers seat on the right sideOne of the trickiest aspects of adopting different driving rules from one country to the next is adjusting to driving on the other side of the car. Sitting at the central point in traffic allows the driver to better judge space when passing oncoming traffic and allows passengers to exit on the non-traffic side.The earliest cars were constructed with the driver on the left, ideal for driving on the right, consistent with most horse-drawn carriages and wagons. The earliest cars had their controls on the ground, so it was easier for the driver to manage the controls with their right, usually dominant, hand. This configuration became standard in the United States with the success of the Henry Ford Model T, released in 1908.Right-hand drive cars were imported from the US, but manufacturing quickly adapted, and by 1916, even the Model T was being made with left-hand drive controls for export to the UK, Australia, and other countries. By the 1920s, it was standard for left-hand traffic nations to use cars specifically built for left-hand driving.The Tradition Has Stuck in Britain and IrelandThe United Kingdom and Ireland as seen from the airOver the 20th century, many countries that drove on the left switched to driving on the right to be consistent with their neighbors. Sweden in 1967 and Iceland in 1968 were the last European countries to switch.The United Kingdom seriously considered the possibility of switching in the 1960s, but the cost of redesigning and remarking roads and moving traffic lights, plus the existing car inventory, made the switch prohibitive. Further, there was minimal support for the move among the populace of the proud island nation.