Now that we have user-contributed airports with buildings in X-Plane 10.25, I have a few topics to cover regarding airport authoring. The first is taxi routes. (I will get to the contribution and sharing process in a few posts - please bear with me.)
The short of it is: if you do not provide taxi routing in an airport layout, X-Plane will generate a taxi route, and the route X-Plane generates is not particularly good. The algorithm is slow, so it hurts loading time, it produces a relatively ugly layout, often with errors, so the AI behavior is bad, and I suspect that sometimes the whole algorithm crashes.
While all of these things can be improved, there will always be a trade-off between load time and algorithm quality. For example, the layouts could be rendered using more memory, but the loading would be even slower.
And we don't want to be generating AI layouts - we want this data to be in the apt.dat where it can be built by smart humans who can carefully author.
For example, consider an airport with a huge tarmac, where the taxi routes are simply painted onto a big slab of concrete. The AI algorithm doesn't understand this line structure at all - it isn't smart enough to 'get' the taxi instructions. So the AI network lets the plane drive straight across the slab of concrete, traffic flow be damned! The AI also has no idea what any taxiway is named, so ATC can't provide good route names.
The pictures above show the actual generated taxi layout at KJFK - red indicates active segments where the aircraft hold short before crossing runways. Nerds will recognize that this layout is created by a straight skeleton erosion algorithm, a strategy for analyzing images that comes from OCR.
The moral of the story is: while the AI taxiing behavior can be quirky, a lot of the problems come from X-Plane trying to auto-generate layouts off of the apt.dat file without a real taxi route structure. If you create such routes, the sim loads faster, provides better directions, and provides more plausible taxi routes.