Air travels through different objects, but I am very confused because you see these pictures of cars doing wind tunnel tests and that smoke goes over the car to represent the cars aerodynamics. When you are driving in a real situation does some of the air travel through the windshield and if it does then why doesn't all air travel through the windshield.
Air does not travel through solid objects, Physically speaking. Air molecules are too big to get through gaps between atom in solid materials. However, that doesn't mean there is no exchanging of air inside and outside of cars. The most obvious example is air conditioning which sucks air from the outside and circulating it in the car.
Typically, when you driving with windows closed, there is an air pressure different between outside and inside of your car. Sometime, this different can be large if you are driving through fast moving wind. The windscreen, as well as any other point of your car, is built to withstand in this situation. Pressure different doesn't necessary mean air is leaking into your car unless the windscreen gave way.