The role of longitudinal vortices in the development of aerodynamic forces on road vehicles is discussed. It is shown that the drag and the side force on a vehicle can be determined from a suitable survey of the wake. However the lift, which has a component related to the shed circulation and a component arising from the attractive force between a vehicle and its image in the ground, cannot be estimated in the same way. By studying plots showing the longitudinal component of the shed vorticity in the wake it is often possible to identify a number of different vortices. These vortices are found to originate from different features of the vehicle body.