# Parameter Study for Child Booster Seats in Frontal Collisions
## 1 Introduction  
According to \"Global Road Safety 2010-2018\", traffic accidents are the leading cause of death for 5-20 year olds [1]. In particular, regarding the safety of middle-aged and older children in vehicles, 3-11 year olds account for 42% of child occupant fatalities, with approximately 62% of children sitting in booster seats at the time of the collision [2, 3]. The booster seat is a Child Restraint System (CRS) for middle-aged and older children and can significantly reduce children's injuries during a motor vehicle crash. According to the Washington State motor vehicle crash data between 2002 and 2015 involving children aged 8-12 years, child occupants using booster seats were 29% less likely to be injured than those restrained only by seat belts [4]. The booster seat is equipped with a three-point seat belt to constrain the child occupants, so the study of the general layout parameters of the booster seat includes both the study of the three-point seat belt restraint path parameters and the structural design parameters [5, 6]. Since the three-point seat belt restraint path is designed according to the adult body size, the situation occurs that the shoulder belt is too close to the child's neck and the lap belt slips from the pelvic bone position during the seat belt operation process, resulting in a larger neck and abdominal injury to the child during a collision [7, 8]. Therefore, the restraint path specification of a three-point seat belt is a primary factor in achieving an ideal level of protection [9]. In addition, during the process of analyzing the kinematic posture changes and the injury indices of child dummies during the collision, it was found that the adjustment of +structural design parameters of CRS has a large impact on child occupant injuries [10, 11]. [PERSON] et al. [12] established a frontal collision simulation model for CRS and analyzed the effects of seat belt webbing stiffness variation. The results showed that the seat belts absorb the impact energy, so the webbing stiffness directly affects the injury response of the dummy. [PERSON] et al. [13] analyzed the correlation between the seat cushion angle and injury indices, and between the backrest stiffness and injury indices. The results showed that the seat cushion angle can influence a child's sitting posture before the car crash and kinematic posture in the car crash, while the backrest stiffness affects the deformation of the seat, thereby affecting the response of injury indices. To analyze the root causes of bias during experimental testing of CRS, [PERSON] et al. [14] explored the influence of the initial tension of the shoulder belt by changing the initial position of the shoulder belt. Therefore, the suitable design of the structural design parameters of the CRS can enhance the safety of child occupants [15]. Currently, most studies have only investigated the effects of single or partial parameters on the response of injury indices. However, the overall injury severity is a comprehensive response of different parts of a human body induced by various parameters of the CRS. To obtain those appropriate design parameters, a multi-objective optimization algorithm can be constructed [16; 17]. However, multi-objective optimization and design may not be the best strategy for engineering applications due to its slow convergence and a lack of direct observability in high-dimensional spaces [18]. To accelerate the efficiency of product development, time-consuming multi-objective optimization and design can be transformed into single-objective optimization and design. The single-objective optimization and design with the Weighted Injury Criterion (WIC) as the optimization and design objective can facilitate an efficient search of the best solution [19]. However, the traditional occupant Weighted Injury Criterion (WIC) [20] is designed for adults and considers the Head Injury Criterion, 3 ms chest resultant acceleration, the thoracic compression deformation and maximal axial forces of the left and right thigh.