More Transport would be a good idea. One Way Roads are in my opinion crucial at some point since it would improve the traffic flow.
For Car-Parking for pax, if it is really the case that this is wanted or planned, add a parking garage as outdoor object (like the ATC Tower or the Fuel Station) with a road connection, and let the pax drive the car in there, let the passenger get out of the building and enter the departure hall the car then just “vanishes” vice versa with arrivals, some passengers enter the garage, “summon” a car, and go home by car. Its not necessary to “assign” a car to a passenger, i think a very “shallow” simulation of this is enough to increase reality already. No one really checks each passenger by his name and waits until he comes back
This also can be connected with Car Rental shops, Passenger arrives, gets baggage, goes to car rental, goes to parking garage, and a car with a specified color, or a “advertisment” on the roof from “Hartzi” or “Saxt” appears on the onramp of the garage
and the airport got a little comission fee