Speed-dependent temporal development of driver fatigue in extra-long highway tunnels: An on-road study

OBJECTIVE: Prolonged driving in extra-long highway tunnels is characterized by sustained visual monotony and reduced environmental variability, conditions that trigger the insidious onset of driver fatigue. While fatigue in tunnel environments has been widely discussed, limited research has systematically examined how driving speed modulates its temporal development during extended exposure. This study aimed to investigate the speed-dependent characteristics of fatigue development in extra-long tunnels and to identify corresponding temporal and spatial fatigue-prone intervals. METHODS: On-road driving experiments were conducted in an extra-long highway tunnel under three operating speed conditions (60, 80, and 100 km/h). Driver fatigue was assessed using electrocardiography combined with self-report measures. Within the frequency domain of heart rate variability, the low-frequency to high-frequency power ratio (LF/HF) was adopted as the objective indicator of fatigue. Mathematical models were established to quantify the relationship between the LF/HF change rate and driving time under each speed condition, and a fatigue threshold was determined using the 85th-percentile method. These models and the fatigue threshold were then used to identify the onset of fatigue and the time of peak accumulation. Temporal fatigue indicators were further converted into equivalent travel distances to estimate speed-dependent spatial intervals within the tunnel. RESULTS: Driver fatigue in extra-long tunnels manifested a gradual and cumulative progression, characterized by a significant temporal increase in the LF/HF change rate. Driving speed significantly influenced the temporal characteristics of fatigue development. Specifically, the time to reach the peak LF/HF change rate increased with speed, occurring at 176.4 s (60 km/h), 204.2 s (80 km/h), and 256.8 s (100 km/h). No significant differences were observed in LF/HF change rates at fatigue onset across speeds, and a common physiological threshold of 1.42 was determined. Converting temporal fatigue thresholds into travel distances revealed speed-dependent fatigue-prone intervals along the tunnel. CONCLUSIONS: Driving speed modulates the temporal trajectory of fatigue accumulation without changing the physiological boundary of fatigue onset, suggesting that fatigue-related crash risks in extra-long tunnels are intrinsically linked to operating speeds and should be considered in speed-aware safety management strategies.