There are a lot of good answers here. I’d like to add that the idea of sitting on one side is actually older than automobiles, and that the preference for sitting on the left is based on the fact that most people are right-handed.
A carriage driver cracked a whip with his strong hand, which was usually the right hand. If he’s sitting on the left, his right hand is close to the center of the carriage, and he can reach any horse on either side with equal ease. If he was sitting in the middle, he’d have to reach extra far across his body to crack the whip over the front left horse, which would get exhausting on long drives.
Occasionally, you’d see carriage drivers sitting on the right in old photos. In almost all cases, they’ll be cracking the whip with their left hand.
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