The European legislation for rear underrun protection systems on heavy vehicles, Commission Directive 2006/20/EC, specifies a series of criteria that must be fulfilled so that these devices can be homologated. The different models of devices used as rear underrun impact protection in tank vehicles for fuel transportation are an example of these criteria. Basically, the homologation tests based on this regulation consist in the quasi-static application of a load series at different points of the structure. In analysing the guidelines of Commission Directive 2006/20/EC, a number of questions about their validity may be raised with respect to, for instance, how efficient a homologated system of these characteristics is in real collision. The aim of this paper is to compare the behaviour of a rear underrun impact protection system, incorporated into a tank vehicle, to the behaviour of the same device when a car hits it at different collision speeds.