I: Thou shalt not sitteth next to someone elseth on an empty buseth
A girl is sitting on an empty bus. How tranquil.
Another person boards the bus. Let's call her "Anna". Where will Anna sit?
Do you know?
You should know. Society has rules for moments such as these. Implicit social rules.
Never taught but always learned.
Anna cannot sit next to the other person. That would be weird.
She cannot sit directly behind the person, because she will un-nerve the other passenger.
She cannot sit directly in front of the person, because that would be weird and assymetrical.
And she cannot sit directly across the aisle from the other person, because then they would be sitting next to each other, in a sense.
Under no circumstance must you be conceivably sitting next to the other passenger.
In exceptional circumstances (ie to avoid sun, or get a better view) then she may sit in the back row on the same side, provided she is not directly behind the other passenger (weird).
But it is preferable that she sits next to the window on the opposite side of the bus, provided she is not in the same row as the other passenger (weird).
A few simple rules of thumb will help one choose the correct seat.
1. One should always sit as far away from the other person as possible.
2. If one can mentally draw two lines protruding from oneself, front-to-back and side-to-side, without intersepting the other passenger, then one has chosen correctly.
Now you are ready to go out and enjoy your journeys, free of social awkwardness!
[NB: the above rules should be reversed in collective cultures]