Current design of seat belt systems may provide insufficient protection for the car occupant. Thus occupants may sustain head injuries due to large head excursion. Chest injuries may occur due to unfavourable restraining geometry or excessive belt slack. Finally, spinal injuries may appear due to high vertical loads on the body.Using basic criteria of favourable biomechanical restraining, design improvements of a conventional three-point seat belt system are proposed.It is shown that the design of the seat is of great importance to the biomechanical protective effect of the seat belt. A simple redesign of a conventional seat is proposed. Comparative sled tests with a new seat design and with a conventional seat are performed. The effects of a webbing-locking B-pillar loop and of an automatic belt tightener are also studied.The results show that significant improvements can be obtained by simple means. Thus, with an optimized belt system, the reductions of HIC- and SI-values are found to be in the order of two thirds and one third, respectively. In addition, significant reductions of the vertical components of deceleration and the head excursion are observed.