**5. Discussion**

For ANAS, tunnel lighting is the principal cost item in the energy budget expenditure: at present, the yearly consumption for tunnel lighting is about 230 GWh. Therefore, the Greenlight multi-year project aims to reduce energy consumption for lighting in its road tunnels. It consists of using equipment and technologies characterized by high energy efficiency (i.e., LED luminaries) and balancing environmental-economic needs and the operational limits due to pre-existing structures.

Greenlight aims to improve and standardize safety standards and service to road customers, as well as to optimize energy consumption, especially in more energy-intensive lighting systems. The high CRI, the luminous efficiency and the high visual comfort of LED luminaries allow for significant reductions in the energy consumption with the same luminance on the road surface. For each luminaire considered in the project, the minimum CRI is 80, the minimum luminous efficacy is 105 lm/W, and its CCT is 4000 K. Furthermore, the installation of luminance sensors at tunnel entrances permits the internal luminance to be monitored continuously and verifies the state of efficiency and reliability of the

system. Lighting monitoring is ensured by specific latest-generation software and its results are dealt with in real time. Moreover, in contrast to the HPS luminaries, during day hours this type of luminaire permits point-by-point adjustment of the internal illumination in accordance to the external luminance, in order to optimize the luminous flux. Therefore, the wireless managemen<sup>t</sup> and control system diagnoses the functional status of the individual projectors, and also adapts the brightness of each luminaire to di fferent conditions. During night hours, only permanent lighting remains in operation to guarantee the required luminance level. In order to further reduce consumption, a luminous flux adjustment system is also installed for permanent lighting, which takes into account the reduction of vehicular tra ffic at night.

The benefits arising from Greenlight are related to users' safety, road managemen<sup>t</sup> savings, and environmental protection. ANAS estimates that the return on investment is 7 years, this has been calculated considering only energy savings, without the cost of maintenance. Therefore, the return on investment could be a shorter time because of the durability of LED which is greater than HPS. Considering the costs and the e ffectiveness of the investment, it should also be noted that the knowledge gained to date suggests there is a correlation between the lighting rehabilitation cost and the length of the tunnel. This analysis was presented above (see Figure 3) and this finding might help to expedite the assessment of the cost of road tunnel LED lighting systems: it fits with the variability in lighting rehabilitation costs per unit length that is caused by the luminance requirements laid down by the reference standard UNI 11095.
