There are many different techniques to airbrushing. A person can paint in about any direction that they want. If a car is moving forward, then they would most likely paint in a left to right motion more than a up and down motion. The reason for this is that an airbrush picture can be very detailed because there are many ways to make a picture look realistic. For instance in the picture below they used a bold and white shadow effect. The reason they used this effect is to make it look more like it was outside. And when it comes to realistic in airbrush painting, that's the whole reason to do it.
