What is really happening is that the air slows down as it approaches the front of the Bus, and as a result, more molecules are packed into a smaller space. Once the air stagnates at the point in front of the Bus, it seeks a lower pressure area, such as the sides, top, and bottom of the vehicle. To give a few examples, the worst possible streamlining would be expected from a parachute, which is designed to maximize wind resistance. The Cd of a parachute is about 1.35. The lowest possible resistance is desirable in the airplane wing, which has a Cd of about 0.05. Automobile Cd figures lie between these two extremes. In the past 60 years, automakers have managed to cut Cd figures for production models nearly in half, from about 0.70 to about 0.40. In a practical sense, gas mileage is increased by 5 percent for every 10 percent improvement in aerodynamics.
At speed, the space directly behind the Bus is empty, devoid of air like a vacuum, a concept used for drafting in stock car racing. This empty space is there because the air molecules are not able to fill the hole as quickly as the Bus can make it. Push your hand through water and notice the divot that forms behind it (and notice how the water swells up in front of your hand). The air molecules attempt to fill in this area, but the Bus is always one step ahead. As a result, a continuous vacuum sucks in the opposite direction of the Bus, and this inability to fill the hole left by the vehicle is technically called Flow Detachment, where the air is yanked away from the car.
Now, back to the Mercedes. As the air flows over the hood of the car, it loses pressure, but when it reaches the windshield, it again comes up against a barrier and briefly reaches a higher pressure. The lower pressure area above the hood of the car creates a small lifting force that acts upon the area of the hood (sort of like trying to suck the hood off the car). The higher pressure area in front of the windscreen creates a small (or not so small) downforce and drag. This is like pressing down on the windshield and slowing the car, while the front end is lifted upward.
Where most road cars get into trouble is the fact that there is a large surface area on top of the car's roof and underneath the car, like with our example. As the higher pressure air in front of the windshield travels over the glass, it accelerates, causing the pressure to drop. This lower pressure literally lifts on the car's roof as the air passes over it, while the air passing underneath the car adds additional lift; all of this is a tight-wire act, balancing between too much lift and too much drag. The end result is aerodynamic efficiency.
Colin Chapman, an engineer who invented a new concept to provide downforce without altering drag, called ground effect (different from the aftermarket products of the same name). He incorporated an air channel into the bottom of his Lotus 72 racer, narrow in front and expanded toward the rear. Since the bottom is nearly touching the ground, the combination of channel and ground forms virtually a closed tunnel. When the car is running, air enters the tunnel in the nose and then expands outward toward the tail. Air pressure is reduced toward the tail so that downforce will be generated.
The ultimate example of the downforce concept was the Brabham Alfa BT46B, designed by Gordon Murray for the 1978 Swedish Grand Prix, which actually used a cooling fan to extract air from the skirted area under the car, creating enormous downforce and hence amazing handling capabilities. After technical challenges from other teams, it was withdrawn after a single race, and then the use of skirts used to contain the low pressure were banned (replaced even by a stepped floor design). Another banned attempt at creating a low pressure underneath the car was the Chapparal 2J, which was raced in the 1970 Can-Am series and featured a skirt around the side of the car to stop the outside air from rushing in to break the low pressure created by the top-mounted "extractor" fan. The list of innovations and innovators continued over the years, such as Robin Herds' March 701 and Peter Wright's Lotus 78.
Ground effect is not too suitable for road cars. It requires the bottom to be very close to the ground to form the closed channel. McLaren F1 followed Brabham's lead by using two electric fans to create ground effect, but it is difficult to determine its effectiveness.
It wasn't until the 1960s that automakers noticed that if they reduce the slope of the back of the car to 20 degrees or less, the airflow will follow the roofline and drop off the back of the car, greatly reducing drag. The term for this design was called Fastback, and an excellent example is the Porsche 935/78, better known as the "Moby Dick." The Fastback design isn't without its flaws, especially in the area of lift. Because it has a very large surface area (the roof, in effect, has been extended to the bumper) in contact with airflow, it suffers from a low pressure on top of the car all the time. Usually, wings are used to combat this problem. It seems that good drag and good lift are mutually exclusive-you can't have both of them in equal amounts at the same time.
On the Mercedes, once the air makes its way to the rear window, the drop created by the window and the flat trunk of the car leaves a sizable vacuum, a low pressure space that the air is not able to fill properly and quickly. The flow is said to detach, and the resulting lower pressure creates turbulence, which always deteriorates drag coefficient.
However, the three-box design is still better than a design combining a three-box like the Mercedes with a fastback like the Porsche. If the rear window angle is around 30 to 35 degrees, the airflow will be very unstable. In the past, automakers had little knowledge of this and created many cars in this fashion; thankfully most weren't equipped for speed.