  In SAE Level 3 automated driving, taking over control from automation raises
significant safety concerns because drivers out of the vehicle control loop
have difficulty negotiating takeover transitions. Existing studies on takeover
transitions have focused on drivers' behavioral responses to takeover requests
(TORs). As a complement, this exploratory study aimed to examine drivers'
psychophysiological responses to TORs as a result of varying
non-driving-related tasks (NDRTs), traffic density and TOR lead time. A total
number of 102 drivers were recruited and each of them experienced 8 takeover
events in a high fidelity fixed-base driving simulator. Drivers' gaze
behaviors, heart rate (HR) activities, galvanic skin responses (GSRs), and
facial expressions were recorded and analyzed during two stages. First, during
the automated driving stage, we found that drivers had lower heart rate
variability, narrower horizontal gaze dispersion, and shorter eyes-on-road time
when they had a high level of cognitive load relative to a low level of
cognitive load. Second, during the takeover transition stage, 4s lead time led
to inhibited blink numbers and larger maximum and mean GSR phasic activation
compared to 7s lead time, whilst heavy traffic density resulted in increased HR
acceleration patterns than light traffic density. Our results showed that
psychophysiological measures can indicate specific internal states of drivers,
including their workload, emotions, attention, and situation awareness in a
continuous, non-invasive and real-time manner. The findings provide additional
support for the value of using psychophysiological measures in automated
driving and for future applications in driver monitoring systems and adaptive
alert systems.
