is to be found the optical unit 10e1 as associated with a reflector 10e2 arranged on an arm extending the chassis 10a1 on the other side of the final location of the barrier 1 the way in which the machine in accordance with the invention operates is as follows when the run of barriers 1 1 39 1 34 passes in front of the light box 10e1 the beam f is scattered by the metal of the barriers 2 until an orifice 8 comes in front of the beam fig8 at this moment the beam f passes through the barrier 2 and will be reflected off the reflector 10e2 fig8 this corresponds to the location where a support 2 needs to be driven in a support 2 is then introduced into the bracket 10d and the driving device 10c is then actuated as the support 2 is driven in the attachment slot 2a is caused to move downward fig9 at a given instant this slot comes opposite the oblong orifice 8 in the barrier and the ray of light f passes through it detection of this ray f by the reflector 10e2 gives the signal to stop driving fig1 the depth to which the support 2 is driven then corresponds exactly to the anticipated barrier height all that then remains is for the run of barriers 1 1 39 1 34 to be secured to this support 2 it is clear from the foregoing that the barriers in accordance with the invention together with the erection method and machine designed for this purpose make the erection of crash barriers far easier avoiding the troublesome tasks limiting the labor.