The collision rate between small passenger vehicles and large trucks continues to increase, causing severe impact to communities all over the world as a result of the increased casualties' rate in particular the fatal accident rate. The most dangerous event that can happen during such collisions is the intrusion of the passenger compartment beneath the underride of the heavy truck due to improper design of the rear underride protection device (RUPD) of heavy trucks. The main aim of this book is to discuss how to enhance the crash-worthiness parameters of RUPD which complies with Federal Motor Vehicle Safety Standards (FMVSS 223/224) regulations using finite element analyses (FEA) software such as ABAQUS and LS-DYNA. Based on the experimental analyses results using simulation studies shows that the modified design are better and more stable with the increase of the impact speeds from 45km/hr to 63km/hr. At 63km/hr speed the energy absorption increases by 68%, acceleration decreases by 66.74%, and passenger compartment intrusion (PCI) is reduced by 19.77%. This novel RUPD design is being tested under real crash conditions.