Okay, sorry for that useless title, but I couldn't think up how to have it reflect my question in short. I went on a trip to Rome and got there by bus. Along the way, the bus passed a lot of trucks and some of them had some sort of cloth covering their sides (I'm sure you know what I mean). It was normally just pressed against the trucks, more or less. However, when the bus moved in to overtake the trucks, this 'cloth' was sucked outwards. I'm assuming that somehow the air pressure between the vehicles decreases. I was wondering what causes this.