The effects of vehicle package, seat, and anthropometric variables on posture were studied in a laboratory vehicle mockup. Participants (68 men and women) selected their preferred driving postures in 18 combinations of seat height, fore-aft steering wheel position, and seat cushion angle. Two seats differing in stiffness and seat back contour were used in testing. Driving postures were recorded using a sonic digitizer to measure the 3D locations of body landmarks. All test variables had significant independent effects on driving posture. Drivers were found to adapt to changes in the vehicle geometry primarily by changes in limb posture, whereas torso posture remained relatively constant. Stature accounts for most of the anthropometrically related variability in driving posture, and gender differences appear to be explained by body size variation. Large intersubject differences in torso posture, which are fairly stable across different seat and package conditions, are not closely related to standard anthropome tric measures. The findings can be used to predict the effects of changes in vehicle and seat design on driving postures for populations with a wide range of anthropometric characteristics.
Accurate prediction of driving posture is essential for vehicle interior design. Optimal positioning of the controls, displays, and restraint systems depends on a detailed understanding of how and where drivers of widely varying sizes will sit. Early research into the problem of control and seat placement was concerned primarily with improving the comfort of designs rather than predicting how people would respond to particular vehicle and seat geometries (Lay & Fisher, 1940). Beginning in the late 1950s, designers began to use planar and three-dimensional (3D) manikins, based on the pioneering work of Dempster (1955), to assess leg room and control reach (Geoffrey, 1961; Kaptur & Myal, 1961).
Recently, advances in computer technology have led to the development of 3D software models of the entire human body that are increasingly used for vehicle design (Porter, Case, Freer, & Bonney, 1993). These human figure models provide a useful visualization of vehicle occupant size, shape, and position, but the success of design evaluations conducted with these models is strongly dependent on the accuracy of the model posture.
The current study investigates the effects on whole-body driving posture of three variables that are known to have important effects on seat position and eye location (Flannagan, Manary, Schneider, & Reed, 1998; Flannagan, Schnieder, & Manary, 1996; Manary, Flannagan, Reed, & Schneider, 1998). The analysis is intended to provide an understanding of the individual and interactive effects of seat height, steering wheel position, and seat cushion angle on all of the major posture characteristics of interest for vehicle interior design.
This study was conducted in three phases, each of which used different participants and a different set of conditions. The participants were 68 licensed adult drivers who were selected in gender-stature groups spanning more than 95% of the stature range in the U.S. population (Abraham, Johnson, & Najjar, 1979). Table 1 summarizes the participants' stature distribution by phase.
Testing was conducted in a reconfigurable vehicle mockup that allowed the seat height, fore-aft steering wheel position, and seat cushion angle to be varied over a wide range. The seat and control layout, termed the vehicle package, was specified and measured using standard reference points and dimension definitions documented in Society of Automotive Engineers (SAE) Recommended Practice J1100 and other related practices (described in SAE, 1997). Figure 1 illustrates these dimensions in a side view of a generic package. The x axis in the package coordinate system runs positive rearward, the y axis positive to the driver's right, and the z axis positive vertically. …