In recent years the vehicle design field has followed a progressive evolution by focusing its main interest on passive and active safety levels. One of the most injurious cases for the safety of passengers is the crash between a car and a heavy vehicle (truck). Generally, in fact, when such an occurrence happens, the greatest damage is suffered by the passengers of the car. This is because of the considerable structural difference between the two vehicles and the lack of devices to reduce the injuries of the car passengers. In this work, as a consequence of the previous considerations, we deal with the design of a new protective system for heavy vehicles over 3.5 tons. The new rear underrun protective device presented here, compared to the traditional one, can dissipate a greater amount of energy; this reduces the damage suffered by the passengers of a car involved in an impact with a truck. To simulate the behaviour of the new protective device during different kinds of crashes, dynamic finite element method (FEM) analyses have been carried out. The comparison between the new protective system and the standard one has shown that the new designed protective system has better performances in terms of energy absorption and peaks of decelerations.