An empirical investigation of disability glare and visibility level during driving inside very long road tunnels: A case study

•As the disability glare increases, the visibility level is reduced.•With increasing age, disability glare increased, and visibility level decreased.•With increasing longitudinal uniformity, the visibility level increased.•No correlation was observed between overall uniformity and visibility level. Disability glare from lighting systems is considered to be one of the main causes of accidents in road tunnels. The present study aimed to investigate the estimated method of disability glare, the experimental method of visibility level, overall uniformity, and longitudinal uniformity of the tunnel lighting system during driving inside very long road tunnels. The disability glare in different road sections of the tunnel was estimated based on the threshold increment (TI) recommended by the CIE 88–2004 method. For visibility level measurements, a standard 20 × 20 cm barrier with a reflection coefficient of 20% was placed in different road sections of the tunnel, and the barrier visibility level was recorded as a self-reported questionnaire by 40 professional drivers in four age groups. The estimated method was used to compare the disability glare data with the experimental method of visibility level. Also, in order to investigate the effect of overall uniformity and longitudinal uniformity in different road sections of the tunnel on visibility level, according to the IESNA LM-71-96 and IESNA LM-50-99 standards, these factors were also examined. For the age groups of 19, 40, 64, and 80 years, respectively, the TI in the north lane in road sections 5 (17.7%, 18.4%, 28.65%, 44.3%), 8 (28%, 31.3%, 41.2%, 52.7%), and in the south lane in road section 4 (16.8%, 17.4%, 24.2%, 41.3%) had a higher disability glare than the CIE88 standard (TI = 15%). The results showed that visibility levels were reduced in sections of roads with higher disability glare. Also, in road sections 5 and 8 of the north lane and road section 4 of the south lane, longitudinal uniformity was equal to 0.53, 0.57, and 0.5, respectively, which is less than the CIE88 recommended (0.6),Therefore, when comparing longitudinal uniformity with visibility level, it showed that visibility decreased significantly in these road sections. However, although overall uniformity was reported to be less than standard in most road sections, no correlation was observed between overall uniformity and visibility level. The estimation method also showed that the level of disability glare in drivers’ increased with age, w