I am NOT referring to roads and traffic outside the airport, I am referring to roads in the airport grounds. I"m talking about the roadway to the arrival and departure terminals.
I don't see where's the difference, or how this would contradict in any way what I've said about vector roads.
You started by saying "I am referring to is produced by Ultimate Terrain X", and that IS A VECTOR product, and FSX will automatically populate such roads with cars, according to the road type and population density in that area.
As I've said, if the vector roads that comes with the default scenery OR with Ultimate Terrain (doesn't matter where they come from) goes very close to the airport, or like in this case, they also go INSIDE the airport perimeter, they will be populated by such traffic.
Since an addon airport excludes everything from the default scenery (or any vector add-on product) inside the airport perimeter, those cars will stop way earlier.
The obvious reason for this, is that we try to reserve as much memory and fps as possible for the scenery itself, and take under control what is displayed inside the airport perimeter.