I am the blue car in this scenario. If I am stopped on the highway and am passed by a car going fast (about 65 mph), I feel my car rock in the left/right direction. I think I understand why this happens: the wake of the red car in the air pushes my car back and forth.

But if I'm moving slowly forward (say 5 mph or less), I don't feel anything when the car passes. I would think that, if anything, the force would be stronger, because the velocity difference is now 70 mph. I'm moving slowly enough that the rocking would still be quite noticeable. Why don't I feel the rocking any more in this scenario?