Observational Studies of Car Occupants’ Positions

The use of current seat belts has been shown to be effective in reducing deaths and serious injuries to restrained car occupants by 50% compared to unrestrained. Real world accident studies have identified limitations to the performance of set belts. This has led to the next major evolution in restraint design which is the development of the intelligent restraints. The options for intelligent retraints include making the system variable, to take account of occupant age and sex, occupant weight, occupant sitting position (relative to forward structures) and the severity of the collision which is occurring thus changing the characteristics of the seat belt. Data are presented on how a population of drivers and passengers actually sit in cars, and accident analyses will illustrate how injury outcome varies with age and sex for restrained occupants. The implications of the position of the hands on the steering wheel during normal driving and the rotational orientation of the steering wheel during an impact for airbag design are also included. INTRODUCTION Figure 1 Seat Belt Excursion Current seat belts have been shown to be very effective in diminishing the frequency and severity of injuries to car occupants. So much so that high levels of seat belt use are a prime aim of all national transport safety policies in motorized countries. The limitations of the protective abilities of current seat belts have been well documented in many analyses of both field accident data and experimental studies (1). Real world accident studies have identified five categories of limitations to the performance of current seat belts. These are: 1) Head and face contacts with the steering wheel by restrained drivers (2) It is inherent in the kinematics of a restrained occupant that, in a severe collision at a velocity change of around 50 km/hr, the head will arc forwards and downwards, having a horizontal translation of some 60 to 70 ems Figure 1. If a normal steering wheel position is superimposed on such a trajectory, the head and face necessarily will strike the steering wheel. Such contacts usually produce AIS 1 to 3 injuries and are best addressed with the supplementary airbag systems becoming common throughout the new vehicle fleet. Average Cadaver Tests 30 mph Sled Tests Standard Retractor Belt Head Displacement (cm) 2) Intrusion of Forward Structures A seat belt requires a zone ahead of the occupant so that the occupant can be decelerated by the compliance of the restraint system. If intrusion compromises that space, then specific localized contacts can occur. The injury risk from such contacts may well be small if they are occurring with structures which have been engineered appropriately. Indeed, in the ultimate condition, it is better for the occupant to be decelerated not just by the seat belt alone but through a combination of belt loads and contact loads. Those contact loads are through the feet at the firewall, through the knees into the lower dash and through the airbag and belt at chest level. In severe collisions, however, major intrusions are destroying the passenger compartment so that exterior objects are