Saccade dynamics in different spiral tunnels: An investigation of length and radius effects on driver visual load

OBJECTIVE: This study aims to systematically investigate how key geometric parameters of spiral tunnels, specifically tunnel length and radius and travel direction, influence drivers saccadic eye movements and visual load. METHODS: A field experiment was conducted using a wearable eye tracker to record saccadic behavior from 30 licensed drivers. Participants drove through 3spiral tunnels with varying lengths and radii under both uphill and downhill traversal conditions. Four saccade metrics (amplitude, duration, frequency, and velocity) were analyzed using descriptive statistics and ANOVA to evaluate visual workload. These metrics have been selected because they collectively reflect distinct aspects of visual scanning behavior: amplitude indicates the breadth of visual search, duration reflects the time required for processing fixated information, frequency represents the rate of gaze shifting, and velocity denotes the efficiency of oculomotor movement. RESULTS: The findings indicate that tunnel geometry and travel direction significantly affect saccadic dynamics. Longer tunnels and smaller radii resulted in increased saccade amplitude, prolonged duration elevated frequency, and reduced velocity, suggesting heightened visual processing demand. Furthermore uphill traversal consistently produced larger amplitudes, longer durations higher frequencies, and slower velocities than downhill traversal across all tunnels, revealing a directional asymmetry in visual load. CONCLUSIONS: This study demonstrates that spiral tunnel design, especially extended length and reduced radius, elevates drivers' visual cognitive load with uphill travel imposing greater demands. The results provide empirical evidence to inform geometry-based design guidelines for optimizing visual ergonomics and improving operational safety in spiral tunnels.