[xbkingx]

Wanting your whippin' arm lined up center makes sense, but it doesn't explain swapping both the side the driver sits on and the side of the road used by the vehicle. If you had free reign on street side, you'd sit on the side opposite your whipping arm because you could pick people up on either side (no safety preference) and would have more control over the whip. These probably changed at different times.

If you're on the left side of the street, picking passengers up on the left sidewalk, you'd want to sit on the left side to watch your whip. But, the driver is always on the side furthest from the curb, which makes me think it's more about making sure you don't hit oncoming traffic and picking up passengers.

Still doesn't explain the side of the street used. I wouldn't be surprised if there was a royal mannerism edict that said something like, "Gentlemen ambling in opposite directions should always pass one another on the left to ensure they may unsheathe their weapon, should the other engage in dishonorable combat."