[aerohead]

I think the Prius has minimum airflow into the engine bay, has minimized air going under the car,as well as a relatively "clean" underside,all of which reduce the static pressure under the car,which typically is responsible for the outflow blast you witness in other vehicles.Generous radii at the leading edges of the car's nose allow the air to hug the sides with an energetic flow,which help fold the wheel-arch outflow rearwards.Additionally,the roofline allows for some static-pressure regain at the rear of the car,unobtainable with other roof designs,which allows pressure there to reach closer to the forward stagnation pressure of the car.Higher pressure,weaker wake.