Why did they use vertical trade lanes?

[Australopithecus]

... oncoming traffic as long as the pilot only turns left or right. ...

Right - outgoing, Left - incoming. So wherever you want to reach, take the right side or forward (and right section of the lane)
The conjunction situation, where you have to cross, remains a mystery to this day.
... There're a few modern theories about traffic without traffic lights and such. Just check it out.
Not sure tho, its not German.
(To my French brothers, just joking.)

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