Automated vehicles operating in level 3 may request the human driver to intervene in certain situations due to system limitations. Unscheduled transferring of control to manual driving will create safety issues as consequences of inadequate situational awareness and sudden increase of driver workload. In this study, we propose and evaluate tactical-level input (TLI) method with a multimodal human-machine interface (HMI) for driver intervention in short-term system limitations. The HMI system consists of touchscreen, gesture, and haptic interfaces enabling bilateral driver-vehicle interaction. TLI along with the HMI capable of multimodal feedback can provide situation-adaptive spatial information which enhance the driver situational awareness in a short time. To evaluate the proposed system we conducted driving experiments involving unscheduled takeover situations in urban environment using a driving simulator. We analyzed driver reaction times, physiological responses including heart rate, skin conductance and subjective workload as well as qualitative feedback comparing with manual takeover. The results show that TLI can reduce driver workload, reaction times, and improve driver behavior. Moreover, 90% of drivers preferred to use TLI method over manual takeover.
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