Probably drunk drivers! :)

Hit F10 and then click "Unoccupy all road nodes" (left center of screen). That will fix it for now.

Thanks :)

I think I've solved this problem after years of being frustrated about it. I'm testing it this way, and so far no vehicles have returned!

Create a dedicated global entrance for your airport's control gate, as shown in the image below.

https://imgur.com/BLIB8Z8

REMEMBER: use only one gate. Multiple entrances often cause problems with "pathfinder"