One of the top 5 bug reports I see: two AI airplanes starting in the same ramp start position, or worse, an AI airplane starting on top of the user's plane. The pictures are often quite spectacular, if not what we intended for the ATC system.
The code that allocates AI start locations is much improved for X-Plane 10.30, so in the next major update, we should get less cluttered ramp starts.
But the code change isn't enough; there's something that you have to do now if you are working on an airports.
You need to provide more than one ramp start! In particular, if there aren't enough ramp starts for a given airport, X-Plane cannot sanely allocate AI airplanes with space between them.
In the past, X-Plane's UI didn't have room to show a lot of ramp starts, so authors would include only a handful. This has been fixed for a while, so I encourage you to add lots of ramp starts to your airports. This will give the AI planes room to park and the user a choice of all of the real-world gates.
Types of Ramp Starts
X-Plane 10 introduced type and equipment codes to ramp starts. One thing Chris found while working on the ATC system was that users were putting ramp starts in silly locations. For example, some airports have ramp starts on the active runway! He had to write specific ATC code to untangle the disaster that happens when an airliner decides that "on the runway" is a good place to park.
X-Plane 10 lets you define your ramp start as a tie-down, a gate, the interior of a hanger, or 'misc'. The misc category was designed for ramp starts that might be interesting to humans (e.g. "in the run-up area for runway 7") that are not appropriate for AI/ATC use.
I am not a fan of ramp starts near the active runway; I figure if a user wants to minimize taxi time, X-Plane lets you start on the runway, and a start anywhere but the ramp is inappropriate for realistic flight. But whatever you make, consider marking real parking spots as gates, tie downs, etc. and the random starts as "misc".
Static aircraft (3-d models of aircraft that don't move) are a nice way to make an airport look busier with less impact on framerate than AI planes. But this presents a problem: if a static airplane is at gate C4 and a user wants to park there, what do we do?
I believe the solution to this is to automatically place static aircraft based on ramp starts. This is not something we have coded now, and I don't know when we will get to it, but my design idea for static aircraft is to have the airport include a lot of start positions and no static aircraft; X-Plane can then place static aircraft at ramp starts based on a rendering setting, with the user picking more static aircraft if hardware permits.
The advantage of this scheme is that (because X-Plane places the static aircraft), X-Plane would know where they were parked and could make sure the static aircraft aren't in the user's spot and the AI aircraft aren't using a spot taken by the static aircraft.
The todo item for authors to someday support sim-placed static aircraft is, fortunately, the same thing authors need to do now to prevent future AI collisions: just make sure that there are a lot of ramp starts, modeling how the real airport works.
Edit: Just to clarify, 10.30 is not scheduled to contain a big pile of ATC bug fixes. The slight improvement in AI ramp start placement that I put in was a one-off bug fix; more ATC bug fixes will come in a later patch.