ORIGINAL: Tall Paul
The effects in ground effect aren't "apparent", they're real. But difficult to exploit. There's a complicated explanation using a "virtual wing" and Prandtl circulation in my old aero book. I never did get too much into Prandtl.
I have used the bow-wave off semis and buses while cruising on my motorcycle. In just the right position, I could back off on the throttle, and not lose any speed, the bow-wave supplying a push.
Absolutely! Since the air is incompressible at highway speeds, it pushes itself (and you) ahead of the vehicle, reducing your power requirements. Good thing, too, since if it were not so, you could have become a Peterbilt hood ornament in the experiment!
Cheers!
Jim