Pupil area variation and visual adaptation of drivers in urban short underpass tunnels: A field study

Visual changes experienced by drivers in tunnel environments are critical to driving safety. However, most existing research focuses on highway tunnels and typical urban tunnels, while the visual characteristics associated with urban short underpass tunnels remain relatively underexplored. This study aims to investigate the variations in drivers' pupil area in urban short underpass tunnels, examine the significant differences across tunnel zones, and clarify their implications for driving safety.

This field experiment is conducted in three urban short underpass tunnels, involving 35 participants. Continuous pupil area data are collected throughout tunnel, and the speed of pupil area change is calculated. A Boltzmann regression model is applied to fit pupil area changes at tunnel entrances and exits. In addition, the levels of dark and light adaptation are evaluated.

Drivers' pupil responses begin to change during the downhill zone leading into the tunnel, approximately 3 to 4 s before reaching the entrance portal. Inside the tunnel, pupil area does not remain stable but instead increases and then rapidly decreases. The dark adaptation process is longer in distance but involves smaller changes in pupil area compared to the light adaptation process, which occurs over a shorter distance but with greater variation.

Environmental transitions in urban short underpass tunnels significantly affect drivers' pupillary responses. When designing or improving the tunnel safety, special attention should be given to the downhill zone and the zones near the tunnel exit.