Trivial Pursuit
"It's that trivial that we haven't yet managed it with a vehicle that basically only travels in 2 dimensions on specially prepared strips of land where all said vehicles are going the same way."
Actually, Google has managed it, in California of all places -- which speaks to the next point.
With cars, you are restricted essentially to not two dimensions, but one half-dimension: the road, with very limited options for changing or reversing direction. Then you're further constrained by the most difficult part for autonomous cars: traffic. Your tiny half-dimension is populated by other vehicles going different speeds which often move irrationally (sometimes to the point of NOT going the same direction.) Add to that the inconsistencies of road traffic management signs and symbols (especially across different countries), and you have a much more complex problem than that of autonomous flight.
With aircraft, you have three complete dimensions, global mapping and GPS technology, not to mention a relatively consistent system of identifying and communicating with traffic which has been in constant use and improved for over a half century.
The US military has had autonomous and semi-autonomous drones for YEARS now. It's not just a trivial exercise, it's a fact.
Oh, and obligatory pedantry: Kool-Aid is a brand name, spelled as shown and is a registered trademark of Kraft Foods, Inc.