Quantifying drivers’ visual strategies in tunnel approaches under foggy and nighttime conditions using saccadic measures

Objective This study aimed to quantify drivers’ saccadic visual strategies during tunnel approaches under combined adverse weather and lighting conditions. Specifically, it investigated how fog and nighttime illumination independently and interactively affect saccade duration, frequency, amplitude, and velocity, and whether foggy nighttime conditions produce a distinct visual-search pattern.Methods A repeated-measures field experiment was conducted with 30 experienced drivers on a mountainous expressway. Drivers’ eye movements were recorded using a wearable eye-tracking system during a 222 m tunnel approach segment under four experimental conditions: clear weather during the day, fog during the day, clear weather at night, and fog at night. Two-factor repeated-measures ANOVA was performed to evaluate the main and interaction effects of weather and lighting conditions on four saccadic measures.Results Fog and nighttime conditions significantly affected all four saccadic measures. Compared with clear daytime conditions, foggy nighttime conditions produced the longest saccade duration (41.57 ± 3.95 ms), the highest saccade frequency (2.35 ± 0.14 Hz), the smallest saccade amplitude (3.34 ± 1.15°), and the lowest saccade velocity (117.46 ± 18.68°/s). Significant interaction effects between weather and lighting were observed for saccade duration and velocity, indicating that the influence of fog depended on illumination conditions. The combined degradation of visibility and lighting resulted in slower and more constrained visual reorientation.Conclusions Foggy nighttime tunnel approaches induce a distinct visual-search strategy characterized by slower, more frequent, and spatially localized gaze shifts. Under compounded adverse conditions, drivers appear to concentrate visual sampling on the central forward roadway area while experiencing reduced efficiency in gaze reorientation. These findings highlight the importance of continuous visual guidance, enhanced entrance delineation, advance warnings, and weather-responsive safety management for tunnel approaches under foggy nighttime conditions.