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Residents of Samoa are bracing for chaos this month as the Pacific island nation becomes the first country in decades to order motorists to start driving on the opposite side of the road. On the morning of Sept. 7, drivers will switch from the right side of the street  where about two-thirds of the world's traffic moves  to the left, in order to open the nation to low-cost used autos from left-driving Australia and New Zealand. It will mark the world's first road switch since Ghana, Nigeria and Sierra Leone changed sides in the 1970s, and one of the only instances of switching from the right to the left; virtually every other change has been the reverse. Worried about increased accidents, tens of thousands of Samoans have protested the plan. As a Samoan lawyer opposed to the switch told the Times of London, "Cars are going to crash, people are going to die, not to mention the huge expense to our small country."

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It remains a curiosity and a bit of a historical mystery why the world is divided over something as basic as which side of the road to drive on. The fact that most people are right-handed has a lot to do with it; that's why, for much of history, travelers have stuck to the left. Ancient Romans using chariots are believed to have held the reins with their right hands and a whip with their left; to avoid whipping oncoming drivers, they favored the left-hand side of the road (called "left-hand traffic"). It's also easier for right-handers to mount a horse from the left, so riders gravitated to that side to avoid oncoming traffic as they climbed on and off. Finally, knights and other armed travelers favored the left so they could do battle, if necessary, with their good hand.

(Read the 1962 TIME article "Western Samoa: Coming of Age.")

So why does most of the world travel on the right side today? Theories differ, but there's no doubt Napoleon was a major influence. The French have used the right since at least the late 18th century (there's evidence of a Parisian "keep-right" law dating to 1794). Some say that before the French Revolution, aristocrats drove their carriages on the left, forcing the peasantry to the right. Amid the upheaval, fearful aristocrats sought to blend in with the proletariat by traveling on the right as well. Regardless of the origin, Napoleon brought right-hand traffic to the nations he conquered, including Russia, Switzerland and Germany. Hitler, in turn, ordered right-hand traffic in Czechoslovakia and Austria in the 1930s. Nations that escaped right-handed conquest, like Great Britain, preserved their left-handed tradition.

The U.S. has not always been a nation of right-hand drivers; earlier in its history, carriage and horse traffic traveled on the left, as it did in England. But by the late 1700s, the theory goes, teamsters driving large wagons pulled by several pairs of horses began prompting a shift to the right. A driver would sit on the rear left horse in order to wield his whip with his right hand; to see opposite traffic clearly, the teamsters traveled on the right.

One of the final moves to firmly standardize traffic directions in the U.S. occurred in the 20th century, when Henry Ford decided to mass-produce his cars with controls on the left (one reason, stated in a 1908 catalog: the convenience for passengers exiting directly onto the curb, "especially ... if there is a lady to be considered"). Once these norms were set, many countries eventually adjusted to conform to the right-hand standard, including Canada in the 1920s, Sweden in 1967 and Burma in 1970. The U.K. and former colonies such as Australia and India are among the Western world's few remaining holdouts. Several Asian nations, including Japan, use the left as well  a possible legacy of samurai warriors who wore their swords on their left and didn't want to bump anyone  though many places use both right-hand-drive and left-hand-drive cars.

Despite widespread opposition to the changeover in Samoa, the government insists it's prepared for the move. Officials have added road humps to slow traffic and, according to the Wall Street Journal, set up a training area near a sports stadium where people can practice driving on the flip side. Sept. 7 and 8 have been declared national holidays to help people ease into the new law. Leau Apisaloma, a village chief, told the Journal there's no cause for alarm: "In the beginning, it will be hard, but we'll learn  we're not stupid."

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