[graydonengineering]

It makes sense to me that the more side you block off, the more resistance you will have. You have the ground all the time and in the case of a tunnel, you have all sides. If you had an air dam just off the wall surface on the side of your car for example, you would be pushing extra air because that air that could otherwise flow around the side can only go up. If you are driving further from the wall, you will have to plow through side turbulance as the air bounces off the wall and back toward the side of your car. As with all real world cases, there are enless variables you can throw into this but in general it seems to me like driving allong a wall would add resistance, more and more as you get closer to it. Something I never really thought about till now!