Influence of steering wheel stiffness and road width on drivers’ neuromuscular stiffness

Abstract

New technologies like electric power steering and steer-by-wire have made it possible to freely shape the steering wheel dynamics. As the driver is part of the closed-loop vehicle steering system, his neuromuscular response should be taken into account when shaping steering wheel dynamics. It is known that drivers adapt the dynamics of their arm to different traffic situations (e.g., increased neuromuscular stiffness for narrow roads) and to the steering wheel dynamics itself. Based on that knowledge, this research study investigates whether steering wheel stiffness can be used to assist driving behaviour for different road widths. It was hypothesised that with higher steering wheel stiffness (KSW), drivers would keep the combined dynamics constant by decreasing their neuromuscular stiffness (KNMS). In a critical traffic situation like when a road narrows, high KSW could then allow drivers to relax more, as they would no longer need to increase their own KNMS. Eleven subjects took part in a driving simulator experiment where driving criticality was manipulated by alternating wide and narrow straight segments. Three steering wheel settings were tested: baseline, high and ‘adaptive’ stiffness, which changed gradually from baseline KSW on wide roads to high KSW on narrow roads, and vice versa. Against expectation, no decrease in KNMS was found for higher KSW. Adapting KSW to road width, similar to how drivers adapt, did thus not yield the expected benefits in terms of KNMS. In addition, even with high KSW, performance remained worse on narrow compared to wide roads. However, effort was lower with high KSW, while performance was maintained or even increased. Also, subjective ratings were highest for adaptive KSW, which enabled drivers to keep effort constant between road widths. While no effects on KNMS were found, adapting KSW can thus be a useful tool in assisting driving behaviour in different road conditions.