Design implications of drivers’ engagement with secondary activities during highly-automated driving – a longitudinal simulator study

Highly-automated vehicles will provide the freedom for drivers to engage in secondary activities while the vehicle is in control. However, little is known regarding the nature of activities that drivers will undertake, and how these may impact drivers’ ability to resume manual control. In a novel, long-term, qualitative simulator study, six experienced drivers completed the same 30-minute motorway journey (portrayed as their commute to work) at the same time on five consecutive weekdays in a highly-automated car; a system ‘health-bar’ indicated the overall status of the automated system during each drive. Participants were invited to bring with them any objects or devices that they would expect to use in their own (automated) vehicle during such a journey, and use these freely during the drives. Inclement weather (heavy fog) on the penultimate day of testing presented an unexpected, emergency 5.0-second take-over request (indicated by an urgent auditory alarm and a flashing visual icon replacing a system ‘health-bar’). Thematic video analysis shows that participants were quickly absorbed by a variety of secondary activities/devices, which typically demanded high levels of visual, manual and cognitive attention, and postural adaptation (e.g. moving/reclining the driver’s seat). The steering wheel was routinely used as a support for objects/devices. Drivers were required to rapidly discharge secondary devices/activities and reestablish driving position/posture following the unexpected, emergency hand-over request on day four. This resulted in notable changes in participants’ subjective ratings of trust on the final day of testing, with some participants apparently more sceptical of the system following the emergency hand-over event, whereas others were more trusting than before. Qualitative results are presented and discussed in the context of the re-design of vehicles to enable the safe and comfortable execution of secondary activities during high-automation, while enabling effective transfer of control.