Legend has it that Napoleon objected to the time-honored military practice of marching on the left side of the road with weapons at the ready in the right hand: it put lefties like him at a strategic disadvantage. Once in power, the story goes, the French emperor whose queen, Josephine, was also a southpaw ordered his armies to switch sides. Civilians in countries he conquered had to do the same. Hence, supposedly, the rules of the road as we know them were born, which also explains why the British (who, along with the Prussians, defeated Napoleon at Waterloo) still drive on the left.
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