Investigations on the required luminance in tunnel entrances

TLDR

The study bases tunnel entrance luminance on drivers’ visual adaptation, approximating it by the prevailing entrance luminance and the surrounding veiling luminance. The required entrance luminance is derived by ensuring a 10‑foot target of specified contrast remains visible despite straylight, with further subjective tests in simulated scenes to refine the contrast criterion. Results show that a 10‑foot target at contrast 0.25 yields fair‑to‑good safety, leading to a rule of thumb that entrance luminance should be about twice the veiling luminance, while a lower safety level (contrast 0.3) permits reducing it to two‑thirds.

Abstract

The method described to derive the luminance in the entrance zone of tunnels has been based on physiological considerations of the adaptational processes taking place when approaching a tunnel. The driver's adaptation can be sufficiently approximated by the luminance prevailing in the entrance he is fixating and the superimposed equivalent veiling luminance L seq caused by the surrounding field. The required luminance level in the entrance zone can be obtained by postulating that a 10' object of a defined contrast should be visible as the straylight reduces the contrast in the eye of the approaching driver. Additional investigations were carried out in which the observer had to evaluate subjectively the entrance luminance in simulated tunnel scenes because it was uncertain what contrast of the object could serve to sufficiently describe the visual task. It was shown that levels rated to provide between fair and good safety conditions almost coincided with the visibility of 10' target with C = 0.25. With this criterion the required entrance luminance L 2 results as a rule of thumb to be twice the measured or calculated veiling luminance Lseq. A decrease of safety demands to "fair" nearly corresponding to C = 0.3 would allow L2 to be reduced to two-thirds of that value.