> Also, autonomy in the sky seems easier than autonomy on the ground.
This is debatable.
A car has only 1 degree of freedom. An aircraft typically has 3 degrees of freedom.
A car rests on solid ground. An aircraft is submerged in a fluid medium.
A ground vehicle can adjust it's speed to the conditions, if necessary slow down if confidence levels get lower, or even go backwards. An aircraft is typically subject to an aerodynamic envelope.
As for the sky being empty, take a look at this harrowing story of a biz jet encountering the wake turbulence of a A380. Airframe written off, crew earn their lifetime salary. Autopilot took 13 seconds to conk out.
Turns out modern aircraft following airways to a precision of mere meters has increased such incidences. In the olden days pilots where often a bit off the track. To simulate this sloppiness in flying rigor, a thing called SLOP has been conceived, for Strategic Lateral Offset Procedures.
Also, autonomy in the sky seems easier than autonomy on the ground.
If Thrun is involved, chances are it will work pretty well...