A support frame attaches a truck mounted attenuator to a backup vehicle. The support frame includes a rear section connected to a front section by a linkage which includes two spaced side frames. The side frames each include three hinges, with the intermediate hinge arranged to fold outwardly in an impact. The hinges pivot about vertical hinge axes such that the front section is supported during pivoting of the hinges. The support frame is held in an initial configuration by restraining cables connected between the linkage and the front section. The cables are releasably secured to the front section by a trigger mechanism which is activated by the position of a probe mounted to the TMA.