I have to disagree. If you have a room that has air (from a fan or the wind) blowing into it, the room will pressurize and once it reaches equilibrem there will be no flow. If on the other hand you open a window on the other side of the room you will have some flow thru the room. The amount of flow will be decide by either the opening in the inlet or the outlet.
My point is that a lot of driving is done at a speed where the fans are not pulling more air than the airflow of driving is providing and a poorly designed shroud will block air flow.
