Directional tactile alerts for take-over requests in highly-automated driving

One of the most significant safety concerns regarding Highly-Automated Driving (HAD) is drivers’ ability to regain control of the vehicle safely. Vibro-tactile alerts were already suggested as an effective modality for Take-Over Requests (TOR) in terms of reducing reaction times. However, it is not clear yet whether such alerts should be compatible or incompatible with the location of hazards that might be present when the TOR is initiated. Studies regarding tactile directionality in other domains, and in manual vehicles have found mixed results. It is argued that part of the contradictory evidence may be related to contextual differences between the driving domain and other domains. Thus, this study aimed to test which directional design would be preferable for TORs in time-critical situations. Twenty-seven participants drove a highly-automated vehicle on a highway with two lanes in each travel direction, in a driving simulator. Each participant experienced five TORs in which they were required to take control and divert their vehicle away from an impending hazard that shut down an entire lane and was situated four seconds ahead. The disengagement of the autonomous driver was signaled using a tactile alert. For the first group, the tactile alert was directed towards the hazard (incompatible with the required action), for the second, it was directed away from it (compatible with the required action), and for the control group, the alert was non-directional. Results showed that drivers using the compatible alert reacted faster and more accurately than those using the incompatible alert. Participants using the non-directional alert reacted slower and less accurately than participants in both directional groups. The results contradict previous findings in the manual driving domain, where drivers are faster and more accurate to respond when the alert is compatible with the location of the hazard and not with the direction of the required action. It is argued that these discrepancies stem from the modified HAD driving task demands where drivers are disengaged from the driving task for long periods and are less aware of the driving environment. The implications for the design of autonomous vehicles are discussed.