Driving on the right came about by whim. As long ago as the days of knights on horseback, they - being mostly right-handed, as we mostly are today - approached each other right side to right side. This was so that they could wield their swords effectively if the need arose. In other words, they rode "on the left".
Driving on the left was the rule in Roman times, too. There is archaeological evidence for this in the form of excavated cart-wheel ruts at the entrances to quarries. One pair of ruts is deep - obviously created when the carts emerged laden with stones - and the other shallow, where they entered unladen. The deep ruts prove they drove on the left.
Later, it is recorded that London City Council appointed three officials in 1722 to keep traffic on London Bridge on the left.
Napoleon imposed the drive-on-the-right rule in Europe for no other reason than to stamp his authority...that was the original whim.
Virtually all early automobiles, created by the pioneers both in Europe and America, had the steering-wheel on the right. Just like the knights of old, it made sense, therefore, to drive on the left. By doing so, the driver could easily see to manoeuvre his vehicle past approaching traffic and could judge when to overtake without endangering himself.
What happened to change things was that Henry Ford suddenly started mass-producing cars with the steering-wheel on the left. So, just as it had made perfect sense to drive on the left before, it now made perfect sense in the USA to drive on the right. Why did he put the steering-wheel there, when practically no-one else had ever done so and nothing was forcing him to buck the trend in Napoleonic style. Well, obviously whim...nothing more...sheer whim once again! Two whims made a right, you could say!
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