Its pretty simple if you think about it.
Your doing 70 MPH down the freeway. Therefore you've got an air flow of 70 MPH in to the radiator (assuming a day of no wind), and your cooling fan is going to try and oppose that air flow by blowing the opposite way! I don't think so!. cooling air is ALWAYS blown from the front to the back of the engine bay. Irrespective of wether the fan is mount in front, or behind the radiator.

Rob