Driver Mental Stress in Response to Thermal Stress Change during Highway Driving

Monitoring driver thermal stress is an integral step for developing an automated climate control function. In this experimental study, various physiological measures for driver’s thermal stress were tracked while intentionally by altering thermal conditions of the seat with a seat air conditioning system (ACS) in summer and a seat heating system (HS) in winter. It was aimed to determine reliable physiological measures for identifying the changes in thermal status induced by the two seat climate control systems. In the first experiment, twenty experienced drivers drove a comfortable sedan for 60 minutes on a real highway while varying the intensity of the seat ACS every 10 minutes to incur ‘hot’ – ‘cool’ – ‘hot’ – ‘cool’ thermal stress. In the second experiment, a new group of eighteen drivers drove the same highway for 30 minutes while increasing the intensity of seat HS to incur ‘cold’ to ‘warm’ thermal stress. Their thermal stress status has been evaluated by heart rate variability (HRV), skin conductance (SC) level, heart rate (HR), and respiration (RES) rate, as well as subjective discomfort ratings during driving. The reliability of each physiological measure was determined by detection rate, which indicated the ratio of occurrences that the physiological measure followed the changes in thermal conditions. The thermal change by seat ACS was detected over 60% by the high-frequency power of HRV, mean SC level, and RES rate. Changes in the thermal stress by seat HS were detected over 60% by the low-frequency power of HRV and RES rate. The findings of this study suggest that monitoring the driver’s HRV and RES rate may enable the vehicle to detect the changes in the driver’s thermal stress reliably.