While we have custom models, their logic is controlled by the simulator itself, we just set the path they should pass through.
The reason why they stack up, it's because the path passes too close to your airplane, but that of course how the real path is, it's just the default strategy of the sim that works this way, and it's too conservative, making the vehicle stop because it assumes they might crash into your airplane, because the default logic is not smart enough to recognize the vehicle will likely pass under the wings or the tail.
Of course, as explained in the KORD V2 manual, Page 12, those vehicles can be disabled with the "Ground Traffic" option in the Addon Manager.
