## 4 Discussion  
Numerical models along with tests on human cadavers and specialized physical pedestrian dummies play a pivotal role to reflect the VRU impact situations. It has been highlighted that in VRUs impacts, head injuries are one of the most common injury types. A number of publications noted that brain deformation or strain is a principal mechanism of the head injury [60; 77]. From the biomechanical perspective, head injuries are regarded as the consequence of a series of mechanical interactions of the complex multi-layered scalp-skull-meninges-CSF-BVs-brain system; the CSF is cerebrospinal fluid and BVs is the abbreviation of bridging veins. However, measuring strain, especially in vivo, during an impact is a big challenge, which also implies ethical issues. Therefore, numerical analysis allowed the authors to verify, among others, VRU head injuries after a collision. This was possible as the head injury criteria are available for FE head models. Some injury tolerance thresholds, which were put forward for the state-of-the-art finite element human model GHBMC (Global Human Body Models Consortium) are presented in Table 3[78]. A proper understanding of the injury mechanism is of uttermost importance for studying injury prevention. Without knowing the proper injury mechanism, the associated injury criteria and thresholds, it is not possible to use numerical models to predict the type, location and severity of head injuries. Beside the criteria presented, there are other injury tolerances thresholds to other biomechanical responses in the literature [79; 80; 81]. The presented study helped to understand the VRU kinematics and head injuries at the tissue/bone level but also is advices appropriate passive safety devices. The models not only enabled the author to assess the risk to injury in vehicle impacts but also are to optimize the VRU in the early stages of the vehicle or safety gear design process at relatively low costs. The comprehension of the factors and variables, which influence the accident would not only help in accident reconstruction but also allow improving the vehicle construction in terms of safety enhancement for VRUs. The process of contact for the VRU involves a combination of slide, roll and bounce to rest--yet the overall injury severity is largely determined on both linear and angular acceleration, which challenges the current HIC/HPC criteria. As it was presented in Section 2, these criteria, which were developed from the Wayne State Tolerance Curve (WSTC), depend exclusively on the translational acceleration without taking rotational acceleration into account [83, 84]. As a result, both revealed peak acceleration and calculated HIC need to be below a threshold, which is defined by regulations for testing or certification of safety devices. Exemplarily, in the presented ECE R22.05 the maximum value of HIC = 2400 is stated, while the peak acceleration needs to be limited to 275 g [85]. Base on the state of the art, the representation of a threshold of severe but not life-threatening injuries HIC(36) = 1000 is widely accepted as an empirical predictor for head injury [86]. The results presented in Table 2 supports these findings. It turned out that for an ECE R22.05 type-approved motorcyclist helmet [50] the intracranial pressure in region of interest (compare Table 3) are much above the threshold. The calculated pressure (>1 MPa) may lead to sever brain injuries. Regardless if the procedure is described by legislation or testing regulation: the background of establishing these thresholds is to avoid Traumatic Brain Injuries (TBI) or skull fractures by the appropriate performance of passive safety devices. Nonetheless, Diffuse Axonal Injury (DAI) and Subdural Hematoma (SDH) are the types of head injuries commonly occurring for the impacted VRUs. DAI is the most common cause of post-traumatic coma and may cause a vegetative state and severe disabilities [87, 88, 89]. As initially remarked, HIC is not considering rotational components of acceleration.