An introduction to tunnel lighting: Basis, calculations, and future lines in the interface between safety and sustainability

The exponential increase in the number of new tunnels, their length, and complexity makes safe and comfortable driving in these infrastructures a must. Among all the technical characteristics necessary to achieve this target, accurate lighting is the most important. However, the peculiarities of driving in tunnels, narrowly linked to the infrastructure itself, but also to physiological and psychological characteristics of drivers, make good lighting complex and highly consuming in terms of energy, financial resources, use of raw materials, environmental impact, and maintenance. The relatively recent introduction of LEDs in tunnels and the new strategies to decrease energy demands and profit from sunlight, whose energy savings can reach 40% in a wide variety of cases, together with the progressive aging of drivers, are challenges for researchers in this field, that currently seek new perspectives affecting the tunnel, the roads before and after, and the portal surroundings. This work approaches the principles of tunnel lighting, its singularities, open points with difficult solutions, and some others that are already contributing to safer and more sustainable tunnels and underground roads.