# Evaluation of Urban Traffic Accidents Based on Pedestrian Landing Injury Risks
## 4 Discussions of Results  
Combined with pedestrian landing kinematics, the biomechanical mechanisms of pedestrian injury under different ground impact conditions is deeply analyzed as follows. * _For PRA 0\({}^{\circ}\)_  
When pedestrians land in the orientation of PFOx+, the feet contact the ground first. Due to the counterclockwise rotational momentum of the body, the pedestrian's knees will bend forward until kneeling. Meanwhile, the upper body leaned backward and eventually the back of the head collided with the ground. In addition to serious head injuries, this landing position would also lead to the risk of waist injury due to excessive reverse stretching of the tissues and organs. When pedestrians land in the orientation of PFOx-, the feet also contact the ground first. Different from the PFOx+ condition, the change of pedestrian kinematics is not only affected by the counterclockwise rotational momentum, but also the front-to-back asymmetry of the body anatomical structure. Restricted by the physiological bending direction of the knee joint, the impact energy (i.e., rotational momentum energy and landing potential energy) suffered by the lower limbs in this landing orientation is much larger than that in the PFOx+. This can easily lead to reverse traction injuries of the knee joint, and also subject the meniscus, tibia, and femur to greater longitudinal impact. Subsequently, the upper body rotates counterclockwise under the residual rotational energy until the head collides with the ground without any body part protection. In conclusion, this landing orientation is prone to serious head injuries, strained knee ligaments, ruptured menisci, and even fractures of the tibia and femur. When pedestrians land in the orientation of PFOy+, the right lower limb becomes the main weight-bearing region with the load of the impact energy (i.e., rotational momentum energy and landing potential energy). It would cause the risk of right knee joint medial collateral ligament strain and tibia fracture. Eventually, the right side of the pedestrian's upper body collides with the ground, which could cause injury to the right shoulder and chest and abdomen organs, but the posture could supply a certain buffer effect on the head-to-ground contact. * _For PRAs 45\({}^{\circ}\), 90\({}^{\circ}\), 225\({}^{\circ}\), and 270\({}^{\circ}\)_  
When pedestrians land at these four PRAs, the torso and lower limbs remained relatively straight during the ground contact, indicating that the impact energy was less absorbed by these two parts. As for the head, when pedestrians land in orientation of PFOx+ and PFOx-, the head collides with the ground directly; that is, the torso provided no protection on the head. However, when pedestrian lands in the orientation of PFOy+, the shoulder could protect the head to a certain extent, which is why the HIC values in orientation PFOy+ are lower than the other two orientations (see Figure 6). * _For PRAs 135\({}^{\circ}\) and 180\({}^{\circ}\)_  
Although the head is the first ground impact region for these two PRAs, the HIC values show significant differences. For PRA 135\({}^{\circ}\), the head has--in addition to its linear velocity component perpendicular to the ground--an angular velocity component that rotates toward the ground. Whereas for 180\({}^{\circ}\), the head angular velocity component is parallel to the ground. Therefore, pedestrian landing at PRA 135\({}^{\circ}\) would result in a higher HIC value than PRA 180\({}^{\circ}\). In addition, when pedestrians land with PRA 135\({}^{\circ}\) in PFOy+, the shoulder has a certain protective effect on the head, resulting in a lower HIC value than the other two orientations. It should be noted that no matter which PFO the pedestrian lands in, the neck bending caused by these two PRAs would be very severe. Especially PRA 180\({}^{\circ}\), although the HIC value of the head is low, the risk of injury is very high due to the extreme bending of the neck caused by the downward pressure of the body.