**3. Methods**

The new lighting systems comply with the Italian standard UNI 11095:2011 Lighting of road tunnels [39]. It divides the longitudinal section of a road tunnel into five zones with di fferent levels of required luminance [40,41] as follows:


equal to the outside luminance through half the length of the threshold zone, and it is reduced in a linear manner to 40% at the end of the second part of the section.


Each zone requires different minimum luminance values as a consequence of the design speed, the meteorological visibility distance, the horizontal lighting in the access zone, the natural luminance, and the optics type, and are calculated using Equations (1)–(3)

$$\mathbf{L}\_{\rm e} = \mathbf{c} \times \mathbf{L}\_{\rm v} = \mathbf{c} \left( \mathbf{L}\_{\rm sccq} + \mathbf{L}\_{\rm atm} + \mathbf{L}\_{\rm par} + \mathbf{L}\_{\rm cru} \right) \tag{1}$$

$$\mathbf{L}\_{\mathbf{t}} = \mathbf{L}\_{\mathbf{t}} / (1.9 + \mathbf{t})^{1.4} \tag{2}$$

$$\mathcal{L}\_{\mathsf{f}} \ge 2 \times \mathcal{L}\_{\mathsf{f}} \tag{3}$$

where Le is the maximum luminance value of Lv; c is a coefficient, which depends on the optics; Lv is the veiling luminance; Lseq is the equivalent veiling luminance; Latm is the atmospheric luminance; Lpar is the luminance of the windshield; Lt is the average luminance value in the transition zone; Lcru is the luminance of the dashboard; t is the travel time in the transition zone; Li is the minimum luminance of the permanent lighting circuit; and Lr is the reference luminance value according to [39]. Lseq is calculated according to the L20 method, which considers the average value of the luminance in a visual cone of 20◦, centred on the line of sight of the driver from the beginning of the access zone.

Since the project involves existing tunnels, no modifications were made to the input data with regard to design speed, meteorological visibility distance, horizontal lighting in the access zone, and natural luminance. Therefore, the stopping distance and the threshold luminance remained the same. The lighting system of each tunnel is composed of one permanent and several reinforcement installations. All devices are arranged in a quincunx geometric pattern in both systems. Counter beam optics create the maximum contrast between existing objects and the road because luminaires are placed above the traffic lanes. The minimum luminous efficacy of each LED luminaire was 105 lm/W and its correlated colour temperature (CCT) was 4000 K; for existing HPS luminaries these average characteristics were 95 lm/W and 2000 K, respectively.

Uniformity performances were settled in agreemen<sup>t</sup> with UNI 11095 [39] and UNI EN 13201-2 [43]; these standards require parameters for night-time and daytime, and for each regulation state of the lighting system, which are:

U0 and *Ut* ≥ 0.50 over the carriageway or over one-way lanes;

*U*0 and *Ut* ≥ 0.40 over all other surfaces and over bi-directional lanes

In areas with constant level of luminance, the standards required are:

*Ul* ≥ 0.70 over the carriageway

*Ul* ≥ 0.60 over all other surfaces

> where:

U0 is the general luminance uniformity, that is, the ratio between the minimum and the average of luminance values [43].

*Ul* is the longitudinal luminance uniformity, that is, the ratio between the minimum and the maximum of luminance values [43] determined along the median axle of a lane for the carriageway.

*Ut* is the transversal luminance uniformity, that is, the ratio between the minimum and the average of luminance values in the same calculus surface section [39].

The software LITESTAR 4D Litecalc [44] was used to design the new lighting systems. It models and verifies the lighting design of road tunnels, and provides numeric and rendered solutions simulating the e ffects of lighting (shadows, reflections, colour vision and rendering volumes) and obtains a picture of the lighted environment. According to the Italian standard UNI 10439:2001 [45], the assumed maintenance coe fficient is equal to 0.8 and the average luminance coe fficient is 0.7. These values coincide with those adopted for designing HPS systems.

The adopted software was proven to be suitable for lighting design, since it has an open database where the operator can save product data, do product research, process photometries and spectra or update data automatically; the software was used to realize a real simulation of the results obtained by means of the designed lighting system.

With regard to the lighting design criteria, as previously stated, the luminaires that constitute the permanent lighting system were placed in the same position of the ones previously installed in the tunnel. Each luminaire is equipped with a luminous flux regulation system (transmission module), which communicates with a control unit placed in the cabin. The reinforcement circuits are managed through the veil sensors placed on the external side of the portals, while the permanent lighting circuits are managed through pre-established time cycles. All lighting devices can be modulated to maximize the energy saving performances.

In spite of the design constraints (especially the fixed position of the luminaires), the results of the simulation were satisfying and met all the standard requirements; in particular, with regard to the uniformity, the calculated values generally vary between 0.80 and 0.95 for *Ul* and between 0.60 and 0.90 for *U*0.

Lighting energy and cost calculation take into account the power regulation during the day and the year, for both variations in external luminance and for energy saving.
