Another observation I have made is the pressure wave from a car extends quite far from it's sides. Standing at the side of the road, even a fairly slow moving car will make the grass sway as it passes even when the grass is a few of meters/yards from the car.
I don't think the air is doing any bouncing. It sure feels like bounce when you pass a semi though!
For what it's worth, here is a graph showing the increase in drag with decreasing ground clearance:
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