The invention provides a distinguishing device for distinguishing, between vehicles passing in front of the device, a heavy goods vehicle (HGV) from a coach including over its entire length windows between a top side member and a bottom side member, the device comprising: a vertical stack of emitters of incident beams towards at least top halves of flanks of at least some of the vehicles; a vertical stack of receivers for detecting the beams that are reflected by the flanks of said vehicles; calculation means for calculating the time that elapses between each emission of a beam and the detection of the corresponding reflected beam in order to establish an image of each vehicle flank; and image processor means for detecting therein the absence or the presence of a top side member.