Experimental design and practice of sunshades in road tunnels to mitigate the “black hole effect”

The "black hole effect" at tunnel entrances, caused by abrupt luminance transitions between exterior and interior environments, often induces momentary driver blindness and significantly increases traffic accident risk. While sunshade structures can mitigate this sudden luminance drop, current design practices lack systematic, quantitative guidance on key parameters such as length and transmittance. This deficiency hinders the establishment of smooth luminance transitions and full resolution of the black hole effect. Moreover, optimizing sunshade parameters typically relies on costly, high-risk real-vehicle eye-tracking experiments unsuitable for educational purposes. To address these challenges, this study developed a safe, economical, and highly operable eye-tracking platform based on a scaled tunnel model. This platform systematically evaluates the influence of sunshade length and transmittance gradients on drivers’ visual adaptation, providing theoretical support for design optimization and serving as a practical tool for traffic engineering education.

A 1:50 scale tunnel visual behavior eye-tracking platform was constructed using illuminance data from full-scale real-vehicle tunnel experiments. The tunnel structure, built with foam panels, employed directional lighting techniques with controllable luminaires to reproduce typical tunnel entrance luminance transitions. To overcome the limitation of fixed sunshade parameters in real tunnels, blue shading films and a segmented multilayer arrangement method simulated different length and transmittance gradient combinations. Fifteen test scenarios were designed, covering sunshade lengths from 45 m to 105 m and various transmittance gradient configurations. A high-resolution imaging device mounted on a remote-controlled model vehicle simulated the driver’s perspective during tunnel passage. Subjects wore Tobii Pro Glasses 3 eye trackers to observe reconstructed driving scenes. Pupil diameter data were collected across three spatial segments—approach zone, sunshade zone, and tunnel entrance zone—to investigate the influence of sunshade design parameters on visual adaptation.

Results revealed that sunshades significantly mitigated the black hole effect by reducing pupil dilation during tunnel entry. Without a sunshade, the average pupil area change rate reached 51.4%. With sunshades, this rate decreased to 20–45%. Longer sunshades (90–105 m) provided stronger buffering, reducing pupil change rates to 20–31%. Crucially, optimized transmittance gradients achieved comparable mitigation at shorter lengths. For instance, a 75 m sunshade using a high–medium–low gradient combination reduced the pupil change rate to 34%, matching the performance of a 90 m sunshade with uniform transmittance. These results confirm that well-designed transmittance gradients can substitute for additional structural length, offering an efficient and economical solution for tunnel entrance safety.

By establishing a scaled-model-based eye-tracking platform, this study quantified the visual buffering effects of tunnel entrance sunshades, overcoming limitations of real-vehicle experiments such as uncontrollable parameters and safety risks. The findings demonstrate that appropriate combinations of sunshade length and transmittance gradient effectively reduce pupil fluctuation and alleviate the black hole effect. Furthermore, under structural constraints, refined transmittance gradient design achieves outcomes comparable to longer sunshades, validating the “light over length” optimization strategy. The proposed platform and methodology not only enhance tunnel visual safety but also provide a replicable, scalable solution for experimental teaching in transportation engineering.