A vehicle air suspension system includes a control unit 32 which is arranged to produce a running measure of the level of cross articulation of the two axles 18, 20 and, if it exceeds a certain level indicating that the vehicle is on rough terrain, to open respective interconnections 34, 36 between the suspension units 24 on opposite ends of each axle so as to reduce the resistance to that articulation. The interconnections are arranged to close when the vehicle speed increases so as to provide roll control.