Investigating effects of temperature and CO₂ on driver drowsiness in the context of conditional automated driving

With the introduction of conditional automated driving, drivers are freed from continuous control but must remain alert for takeover requests. This study examines how in-cabin temperature (22.5 °C, 25 °C and 27.5 °C) and CO₂ (4200 and 1200 ppm) influence driver drowsiness and physiological responses under conditional automated driving. A driving simulator experiment involving 60 participants was conducted, collecting subjective ratings, eye-tracking and physiological data. Results showed that cooler temperatures were associated with lower drowsiness levels compared to neutral temperatures. However, physiological responses may mainly reflect thermoregulation when temperature varies, obscuring drowsiness-related changes. Further, although CO₂ concentration did not significantly affect subjective drowsiness, higher CO₂ levels attenuated cardiovascular and autonomic activity, suggesting CO₂ effects on physiological responses can emerge before conscious awareness. These findings suggest that climate control systems in automated vehicles should balance comfort, efficiency and driver alertness, while physiology-based driver monitoring systems should incorporate environmental data to detect drowsiness earlier.