Some screen captures. I don't have any custom vehicles
You surely have, since (as I've said in my first reply "KLAX has both default and custom AI vehicles" ), they come with KLAX.
Airport vehicle density - 40%, Land & Sea traffic - vehicles - 0%, Ships & ferries and leisure boats - 40%
Since you have both default Airport vehicles and custom vehicles, and your screenshots shows a default vehicle clashing into a custom vehicle, to minimize the chance this happening, it's best if you turn down the default ground vehicles, with the Airport vehicle density slider.
As I've said, they don't have any notion of each other, so there's no way to prevent this entirely from happening, unless you turn off default vehicles entirely.