Drivers’ visual fixation in spiral tunnels: Impacts of geometric parameters and travel direction on cognitive load

OBJECTIVE: This study aims to systematically investigate how key geometric parameters of spiral tunnels, specifically tunnel length and curve radius, interact with travel direction, including uphill and downhill, to influence drivers' visual fixation behavior and associated cognitive load, and to reveal the underlying visual adaptation mechanisms in curved and enclosed tunnel environments. METHODS: A real vehicle naturalistic driving experiment was conducted in three operational highway spiral tunnels with distinct geometric configurations: Nanping (1,330 m length, 1,000 m curve radius), Liuyuan (2,200 m length, 850 m curve radius), and Hankou (4,460 m length, 700 m curve radius). Eye movement data from 30 licensed drivers were collected using a wearable eye tracker. Four fixation-related metrics, including fixation duration, fixation frequency, horizontal fixation deviation, and vertical fixation deviation, were analyzed using two-way ANOVA and post hoc tests. RESULTS: Tunnel geometric parameters exerted significant main effects on all fixation metrics. With increasing tunnel length and decreasing curve radius, fixation duration increased monotonically. For uphill driving, it rose from 490.28 ms to 552.18 ms. Fixation frequency and horizontal fixation deviation decreased significantly. Uphill frequency dropped from 2.85 Hz to 2.05 Hz, and horizontal deviation decreased from 21.64 degrees to 12.09 degrees. In contrast, vertical fixation deviation increased progressively from 13.70 degrees to 21.06 degrees for uphill driving. Travel direction also showed significant main effects. Uphill traversal consistently produced longer fixation durations, higher fixation frequencies, and broader horizontal and vertical deviations than downhill traversal under identical geometric conditions. CONCLUSIONS: Spiral tunnel geometry induces a dual visual adaptation mechanism: horizontal visual tunneling coupled with compensatory vertical scanning for far-field path preview. Uphill traversal imposes higher cognitive load than downhill traversal, revealing a directional asymmetry in driving demands. These findings provide empirical evidence for human-centered spiral tunnel design and offer quantitative benchmarks for driver monitoring and adaptive assistance systems in tunnel scenarios.