The present invention is directed towards an aerodynamic drag reducing device for an articulated road vehicle; the articulated road vehicle comprising a tractor unit and a trailer unit, with the trailer tractor unit pivotably connectable to the tractor unit such that a gap is formed intermediate the tractor unit and the trailer unit when the trailer unit is connected to the tractor unit; the aerodynamic drag reducing device comprising an inflatable enclosure (2)attached to a support framework (10); the support framework (10) being mountable on a rearward facing portion of the tractor unit or on a forward facing portion of the trailer unit, so as to retain the inflatable enclosure (2) in the gap intermediate the tractor unit and the trailer unit; the inflatable enclosure (2) being made of a flexible material; wherein, the inflatable enclosure (2) comprises an outer wall (3) and an inner wall (4) spaced apart and defining an air-receiving cavity therebetween; and, the outer wall (3) and inner wall (4) being curved respectively so that the air-receiving cavity defined therebetween comprises a substantially semi-annular transverse cross-section. The purpose of the aerodynamic drag reducing device is to reduce the aerodynamic drag and hence result in a more fuel efficient articulated road vehicle. Tests have shown that up to an 8% reduction in fuel usage can be achieved by installing such an aerodynamic drag reducing device to a standard articulated road vehicle. Clearly, this can result in significant sayings.