Effects of cabin thermal conditions and road type on driver workload and performance: A driving simulator study

OBJECTIVE: Driver workload is a safety-relevant factor that may contribute to impaired driving performance and an increased risk of traffic injury. Cabin temperature is a modifiable in-vehicle environmental condition, but its influence on driver cognitive workload under different road scenarios remains insufficiently understood. This study examined the effects of cabin thermal conditions and road type on electroencephalography (EEG)-derived driver workload, subjective state, and task performance in a controlled driving simulator experiment. METHODS: Seventeen licensed drivers completed simulated driving tasks under three cabin thermal conditions corresponding to PMV = -1, 0, and 1 and two road scenarios: urban road and expressway. Electroencephalography was recorded during each driving task, and theta, alpha, and beta band powers were extracted. Two ratio-based EEG workload indicators, β/α and β/(θ + α), were calculated. Subjective workload, fatigue, thermal comfort, and driving task error rate were also calculated. Two-way repeated-measures ANOVA was used to examine the effects of thermal conditions and road type. An exploratory thermal human cognitive workload index (TH-CWI) was constructed by integrating EEG-derived workload, subjective workload, fatigue, task error rate, and thermal comfort. RESULTS: Warmer cabin conditions were associated with higher beta band power and higher EEG-derived workload-related ratios, particularly under PMV = 1. Among the two EEG ratio indicators, β/(θ + α) showed a clearer differentiation across thermal conditions than β/α. The thermally neutral condition was associated with lower subjective workload, lower fatigue ratings, and more stable task performance. Road type was associated with differences in EEG spectral activity; higher alpha and beta power was observed during expressway driving than during urban driving. Road type did not significantly affect the ratio-based EEG workload indicators. The exploratory TH-CWI showed the lowest integrated workload under PMV = 0 and the highest under PMV = 1, suggesting that cabin thermal conditions had a more consistent influence on workload-related outcomes than road type in the simulator setting. CONCLUSIONS: This simulator-based study provides preliminary evidence that PMV-defined cabin thermal conditions may influence driver workload-related EEG activity, subjective state, and task performance. Slightly warm cabin conditions were associated with elevated EEG-derived workload-related indicators and less favorable subjective and behavioral outcomes, whereas thermally neutral conditions appeared to be more favorable for reducing fatigue and maintaining stable performance. Road-type effects should be interpreted as differences in EEG spectral activation patterns rather than as direct evidence of higher workload during expressway driving. The exploratory TH-CWI may provide a preliminary multimodal summary of driver state and may have potential relevance for future driver-state monitoring, but further validation is needed.