**1. Introduction**

According to the European Commission, improving energy e fficiency is a broad term. Improvements in energy e fficiency and energy saving are the two key targets [1]. An e ffective energy e fficiency policy could contribute to EU competitiveness and encourage eco-innovation, on the other hand, energy saving is the first option for improving energy e fficiency and is undoubtedly the quickest, most cost-e ffective way to reduce environmental impacts [2]. In the public sector, more efficient lighting can contribute to improving environmental sustainability of our lifestyle [3]. In 2009, 14% of the total electricity consumption in Europe was for lighting, and more than half of this was for public spaces and non-residential applications [4]. In particular, over 70% of existing light sources are based on obsolete technologies that have poor energy performance, and for which a better alternative is available [5].

Public lighting is 2% of the overall value [6] of energy consumption in Italy, however, both renewal of technologies and energy rationalization processes could contribute to meeting the three key targets set by EU on climate and energy (i.e., 20% cut in greenhouse gas emissions, 20% of EU from renewables, and 20% improvement in energy efficiency). Among the various and important functions of public lighting is ensuring safety for the movement of traffic and pedestrians. In order to ensure the minimum safety illumination values on roads, specific standards have been set to define minimum luminance and illuminance levels according to the area to be illuminated. In Europe, the updated standard CEN/TR 13201-1:2014 [7] on road lighting provides far greater potential for dynamic control then the previous standard CEN/TR 13201-1:2004 [8], which results in significant energy saving [9,10]. Lighting costs represent up to 25% of the total budget for managemen<sup>t</sup> of the road network [11], therefore both energy performance indicators and environmental criteria should be adopted when designing smart public lighting systems [12,13]. In particular, safety measures for tunnels and their overall equipment involve a high-energy consumption [14]. The aim of tunnel lighting is to allow traffic to enter, pass through and exit the enclosed section safely. This goal implies that drivers drive into the tunnel without reducing their vehicle speed and are able to see unexpected hazards on the carriageway and stop if necessary [15]. During day time, adequate illumination avoids the "black-hole effect" (i.e., a decrease of the drivers' visual perception) while passing from the outside into the tunnel [16,17]. Therefore, different methods and technologies have been adopted to reduce lighting energy and costs, especially in road tunnels [18–21].

Recently, research has focused on the use of sunlight to reduce energy consumption [22] and related environmental impacts [23]. In some cases, it is possible to use the natural light in an open stretch of the road before the tunnel entrance, that is, a pre-tunnel lighting (PTL) structure is positioned before the access to the "natural" tunnel [24,25]. A PTL is a reticular structure that reduces the luminance in the access zone [26,27]. Tension structures at the tunnel entrance shift the zones with the highest energy consumption; their effectiveness depends on the geometry and orientation, latitude and longitude of the tunnel [28,29]. The construction of these structures implies significant design and construction costs, but the life cycle cost analysis has revealed their economic advantage with respect to the "zero option". Pergolas imply lower energy savings and easier maintenance than tension structures [30]. The introduction of sunlight inside the tunnel with light-pipes [31] or optical fibers [32] is another option for more sustainable tunnels, but this strategy is difficult to use because of the relative position between the sun and light-pipes matrix [33]. When it is not possible to use natural light, the inner surfaces of the tunnel play a crucial role in the tunnel lighting design [34]. For this purpose, the pavement reflection coefficient in the visible spectrum has been recently analyzed as a saving strategy. Salata et al. [16] analyzed four alternative solutions to optimize road tunnel lighting systems with two asphalt pavements with different reflection coefficients and two lighting systems (high-pressure sodium (HPS) luminaries, or HPS and light emitting diode (LED) luminaries). Special asphalts with a high-reflection coefficient reduce the power required, and the energy savings balance their higher installation costs. On the other hand, concrete (light) pavements result in 29% less road tunnel lighting costs compared to traditional asphalt (or black) surfaces; calculations for Italian road tunnel pavements performed by Moretti and Di Mascio [35] have confirmed this. Moreover, the use of concrete instead of bituminous pavements implies a higher level of safety in both ordinary and emergency conditions. Indeed, compared to a flexible pavement, a rigid pavement requires less maintenance activities (which interfere with the traffic flow) [36] and it is not combustible in case of fire [23].

Recently, extensive interest in LED has given rise to new lighting systems in tunnels. LED is the most efficient available technology. Its main characteristics are long life, energy efficiency, environmental friendliness, and zero UV emissions; also, LED illumination produces little infrared light and close to no UV emissions, and excellent color rendering [37]. This technology has been adopted by the Italian government-owned road company ANAS (acronym for Azienda Nazionale Autonoma delle Strade—National Autonomous Roads Corporation, Rome, Italy) to rehabilitate lighting systems in its managed tunnels. In 2018, it launched the Greenlight project, which involves more than 700 tunnel tubes where HPS will be replaced by LED luminaries.

### **2. The Greenlight Project**

The Greenlight project provides for rehabilitation works on road tunnel lighting systems, which consist of replacing outdated high-pressure sodium (HPS) luminaries with state-of-the-art light emitting diode (LED) luminaries. The new devices are equipped in order to wirelessly control the light flow and the energy consumption. The implemented remote control allows adjusting the luminous flux with a point-to-point regulation for each LED luminaire. The wireless managemen<sup>t</sup> and control system aims to adapt the luminance curve to the di fferent conditions of external brightness, and to diagnose the functional status of the individual projectors. Therefore, the luminous flux can be continuously adjusted according to the external luminance for the reinforcement circuits, and according to the daily time bands for the permanent lighting circuits. LED technology makes it possible to optimize the dimming levels up to 15–20% of their initial flow while maintaining the required perceptive conditions and ensuring a significant reduction in consumption. During the night-time hours, the required visibility conditions inside the tunnel are comparable to those of the open-air sections [16]; only the permanent lighting system is necessary to guarantee the required luminance level. To further reduce consumption, a luminous flux regulation system is also installed to manage permanent lighting according to the reduction in tra ffic at night.

Compared to SAPs, which only allow a step adjustment for homogeneous groups of fixtures, this technical di fference ensures considerable savings for the road manager as it reduces costs and allows for operation flexibility in case of the addition or removal of new elements.

Moreover, the project aims not only to reduce consumption and optimize of tunnel lighting systems, but also to raise safety levels. Indeed, the new luminaries enhance the visibility and quality of the diffusion of artificial lighting inside the tunnel. This is due to the higher color-rendering index (CRI) of LED compared to HPS. CRI is a measure of a light source's ability to show object colors realistically or naturally: the higher the CRI, the better the color perception. Statistically LEDs have CRI values between 70 and 90 or more, while HPS have CRI values not higher than 70 [38]. Road lighting can use lighting systems that have a CRI below 70 because color rendering is not a major issue. However, LEDs offer significant color advantages over HPS, eliminating the monochromatic black appearance of objects illuminated by sodium-bulbs. Therefore, LEDs allow a further reduction in the electricity used for lighting in addition to their longer service life (LED devices have warranty coverage up to 11 years of continuous operation).

Of a total of 1900 tunnels under management, the project involves about 700 tunnel tubes in Italian territory. Greenlight has been divided into eight geographical areas (Figure 1) and two phases with the overall cost estimated as € 155 million (Table 1).

**Figure 1.** Greenlight project locations.


**Table 1.** Characteristics of Greenlight.

The first phase started in 2018 and is still ongoing: it involves the 147 tubes with the highest energy consumption per unit of length and the overall estimated cost is € 30 million at the end of 2020. Data and the allotted amounts for phase 1 are listed in Table 2.


**Table 2.** Characteristics of phase 1 Greenlight.

All works consists of:


Therefore, there are no modifications, substitutions, or alterations of the existing position and number of luminaries, in both the interior zone and the adaptation zone. Energy-e fficient equipment and technologies have been planned in order to monitor the systems and to control the functionality of the equipment during its service life.
