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That might stop being true if the air is as crowded as the roads.

As long as they're all outfitted with transponders, and piloting is automated, I don't see that being an issue.

The air won't have pedestrians and bikes!

Para/hang-gliders, small drones, birds. There's plenty of stuff you don't want to hit :-)

Cue legislation to make kites, balloons and RC aircraft illegal