This study investigated human-machine cooperation when driving with different degrees of a shared control system. By means of a direct intervention on the steering wheel, shared control systems partially correct the vehicle's trajectory and, at the same time, provide continuous haptic guidance to the driver. A crucial point is to determine the optimal level of steering assistance for effective cooperation between the two agents. Five system settings were compared with a condition in which no assistance was present. In addition, road visibility was manipulated by means of additional fog or self-controlled visual occlusions. Several performance indicators and subjective assessments were analyzed. The results show that the best repartition of control in terms of cooperation between human and machine can be identified through an analysis of the steering wheel reversal rate, the steering effort and the mean lateral position of the vehicle. The best cooperation was achieved with systems of relatively low-level haptic authority, although more intervention may be preferable in poor visibility conditions. Increasing haptic authority did not yield higher benefits in terms of steering behavior, visual demand or subjective feeling.

Although the benefits of haptic shared control of the steering wheel have been repeatedly demonstrated, longitudinal studies of how drivers adapt to this kind of system are still lacking. The present study addressed this question by comparing two groups of participants in a driving simulator for an extended time period; one group drove with a shared control system and the other drove without. After the practice, all participants drove a final trial with shared control during which a failure of the system occurred. The results show that the evolution of driving performance and the way in which drivers monitored their performance was similar for the two groups. This suggests that the drivers quickly updated their internal model of the steering system dynamics at the sensorimotor level, without further behavioural adaptation afterwards. However, it appears that the internal model was consolidated with repeated use of the system, which translated as a difficulty to compensate for the system's failure. In addition, it appears that drivers did not attempt to maintain a level of task difficulty when steering was facilitated.

Some driving devices are designed to prevent road departures. One such device, motor priming (MP), provides small pulses to the steering wheel towards the lane centre, without correcting the trajectory itself. Compared with the other lane departure warning systems, its higher efficacy has been demonstrated; it is hypothesised that this relies on the action of haptic cues at the sensorimotor level (Navarro, J., Mars, F., and Hoc, J.M., 2007. Lateral control assistance for car drivers: a comparison of motor priming and warning systems. Human Factors, 49 (5), 950-960). The way in which corrective manoeuvres, primed by MP, can be influenced by processes that operate at higher levels of cognitive control, such as risk evaluation, is an issue. Results showed that MP improved all indicators of steering efficiency, starting with reaction times. Risk expectation and situation analysis did not influence reaction times but came into play soon after the corrective manoeuvre was initiated. Thus, although MP triggered the response at the sensorimotor level, higher levels of cognition (symbolic control) quickly modulated the execution of the corrective manoeuvre.PRACTITIONER SUMMARY:This paper showed that corrective manoeuvres following directional pulses on the steering wheel (motor priming) are modulated by risk expectation. The conclusion may be of interest for designers of haptics-based automation such as lane departure warning and lane keeping assistance systems.

The present study compared two distinct approaches to designing driving assistance devices. These devices aim to facilitate steering responses by delivering directional pulses on the steering wheel when lane departure is imminent. In one case, the aim is to prime the corrective gesture through a haptic cue in the direction of the lane centre (motor priming). The other approach consists of eliciting a compensatory reflex reaction by means of a jerk of the steering wheel in the opposite direction. Central to this investigation are the safety benefits of the devices and the ability of drivers to remain in full control of their steering responses. The steering behaviour of 18 participants during near lane departure in bends and in straight lines was analysed. The strength and direction of haptic cueing was manipulated. The results show that drivers were always able to control the direction of the steering response when the haptic cue was delivered. No reflex counteraction was observed, whatever the strength or the direction of the stimulus. The fastest responses were observed when the cue was directed toward lane departure, especially when cueing was strong. However, these did not necessarily lead to the fastest returns to a safe position in the lane when compared with motor priming toward the lane centre. The latter yielded improved manoeuvre execution as soon as the steering movement was initiated. These results are discussed in relation to the sensorimotor and cognitive processes involved in steering behaviour. Their implications for the design of haptic-based lane departure warning systems are considered.