## 2 Materials and Methods
### Pedestrian Safety Regulations  
Since the 1960s, the enhancement in vehicle safety was not only a marketing trick but more importantly, was required by relevant standards and regulations. Twenty years later the first actionswere taken to stop the growing number of fatal road accidents involving pedestrians [21; 22]. The legal aspects of vehicle collisions with pedestrians were first dealt with by the European Experimental Vehicle Committee (EEVC) in the 1980s. In 1988 the so-called Working Group 10 (later renamed to Working Group 17) was established under the EEVC, whose objective was to develop methods and define limits on biomechanical values for a case when a pedestrian is struck by the front of a vehicle [23]. In 2009, based on the experience of the EEVC, the Parliament of the European Union issued Regulation (EC) 78/2009 amending Directive 2005/66/EC on the type-approval of motor vehicles with regard to pedestrian safety [24]. In order to validate the parameters established in Regulation (EC) 78/2009, certified impactors should be used, which reflect those parts of the human body that are crucial during collision.  
Regulation (EC) 631/2009, which specifies the (EC) 78/2009, distinguishes two types of headform impactor models: a child/small adult model and an adult model, which have the form of a rigid aluminium sphere, with a diameter of 165 mm, half of which is covered with a 14.0 mm thick synthetic skin. The total mass of the headform impactor, including instrumentation, is 3.5 kg for the child model and 4.5 kg for the adult model [25]. In order to validate the parameters established in Regulation (EC) 78/2009 it is mandatory to use certified impactors which reflect those parts of the human body that are critical in terms of injuries.  
The simulation of the collision is performed at impact speeds of 35 or 40 km/h and includes the following tests (Figure 3a):  
A legform impactor representing the adult lower limb;  
An upper legform impactor representing the adult upper leg and pelvis;  
Child and adult headform impactors.  
As far as head injuries are concerned, to obtain the type-approval for the tested vehicle the HPC (Head Performance Criterion) shall not exceed 1000 over one half of the child headform test area and, in addition, shall not exceed 1000 over 2/3 of the combined child and adult headform test areas. The head impact zone was depicted in Figure 4a. The HPC for the remaining areas shall not exceed 1700 for both headforms [24]. The HPC Equation (1) is the same as for HIC (Head Injury Criterion), thus:  
\[HPC\ or\ HIC\ =\ \left(\left[\frac{1}{t_{2}-t_{1}}\int_{t_{1}}^{t_{2}}a(t)dt \right]^{2.5}(t_{2}-t_{1})\right)_{max} \tag{1}\]  
Figure 3: (**a**) Visualization of a vehicle-to-pedestrian accident and the use of impactors that represent critical parts of the human body; (**b**) determination of head impact zone—according to Regulation (EC) 78/2009, phase II.  
The magnitude of the linear acceleration observed at the centre of mass of the dummy head during the impact is described by a(\(t\)). The resultant acceleration [g] is of duration \(t\) and \(t_{1}\) and \(t_{2}\) are two time points [s] during the impact, \(0\leq t_{1}<t_{2}\leq t\). HPC or HIC considers only the linear, translational acceleration over this previously defined time window. For (EC) 78/2009 time interval greater than 15 ms is ignored thus we can name it as HPC(15).  
A study of the literature concerning pedestrian safety indicates that the biofidelity of the used impactors for (EC) 78/2099 has not been definitively resolved. It has been proven that the impactors used in tests cannot reliably assess the safety of vehicles with a high-bumper and bonnet reference line (SUVs). Using the impactor raises many doubts and questions [26; 27]. The currently used impactors are not reliable in assessing the safety of SUVs for vulnerable road users. Moreover, it should be noted that the limit values of HPC parameters are not related to the mechanisms of injury.