Rear underrun crashes involving heavy vehicles with rear overhangs represent the most extreme examples of the incompatibility between heavy and light vehicles, particularly passenger cars. This paper describes the design, development and prototype testing of a practical, effective energy-absorbing rear underrun barrier system. This builds on the extensive Work previously undertaken demonstrating the effectiveness of well designed lightweight -but rigid- rear underrun barriers. The energy absorbing unit consists of two light weight steel tubes, containing the energy absorbing glass ñbre reinforced composite tube. The full system exhibits very good force-deformation characteristics, with minimum energy absorption in excess of 40KJ. Testing has included static and dynamic loading, including centred and offset crash testing of the prototype unit to compare the injury outcome with that of a full frontal barrier test of the same vehicle model.