You wouldn't believe if I said it was jousting on horses that is primarily to blame for the left-side driving. And it took a Pennsylvania law enacted in 1782 to make America drive on the right.
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The custom of driving on the left of the road probably dates back to prehistory. It was an early road safety measure. In the days when the chief danger on the roads was a mugging, careful travellers passed oncoming strangers sword arm to sword arm. A stylised example of this custom was the joust where knights charged one another holding a lance on their right. Left-handed knights had a short life expectancy."
I loved the part about left-handed knights. Poor southpaws. Since the French fought the British all the time, they stayed on the left too.
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Pakistan also considered changing form the left to the right in the 1960's. The main argument against the shift was that camel trains often drove through the night while their drivers dozed. The difficulty in teaching old camels new tricks was decisive in forcing Pakistan to reject the change."
You know a lot about a country when camels dictate its transportation policy.
Overall, a great insightful article on driving direction history.