  With increasing automation, drivers' roles transition from active operators
to passive system supervisors, affecting their behaviour and cognitive
processes. This study addresses the attentional resource allocation and
subjective cognitive load during manual, SAE Level 2, and SAE Level 3 driving
in a realistic environment. An experiment was conducted on a test track with 30
participants using a prototype automated vehicle. While driving, participants
were subjected to a passive auditory oddball task and their
electroencephalogram was recorded. The study analysed the amplitude of the P3a
event-related potential component elicited by novel environmental stimuli, an
objective measure of attentional resource allocation. The subjective cognitive
load was assessed using the NASA Task Load Index. Results showed no significant
difference in subjective cognitive load between manual and Level 2 driving, but
a decrease in subjective cognitive load in Level 3 driving. The P3a amplitude
was highest during manual driving, indicating increased attentional resource
allocation to environmental sounds compared to Level 2 and Level 3 driving.
This may suggest that during automated driving, drivers allocate fewer
attentional resources to processing environmental information. It remains
unclear whether the decreased processing of environmental stimuli in automated
driving is due to top-down attention control (leading to attention withdrawal)
or bottom-up competition for resources induced by cognitive load. This study
provides novel empirical evidence on resource allocation and subjective
cognitive load in automated driving. The findings highlight the importance of
managing drivers' attention and cognitive load with implications for enhancing
automation safety and the design of user interfaces.
