A reusable trailer mounted attenuator with a collapsible scissor frame assembly supported by front, intermediate and rear vertical frames. A self-adjusting shock absorber with staged metering is attached to the scissor frame assembly and controls its collapse under impact in a manner that deceleration of an impacting vehicle does not exceed permitted rates irrespective of the weight and speed of the impacting vehicle within the design range. Guides are provided to permit the scissor frame assembly to collapse without twisting and a replaceable strike plate with vertical fins concealed with a cover plate is attached to the rear vertical frame.