# Effects of Sidewall Brightness on LED Lighting Environment and Visual Performance in Road Tunnels
## 1 Introduction  
Road tunnel lighting systems consume a lot of energy, and the power consumption of 24 h continuous lighting in tunnels has become a heavy burden during tunnel operation [1, 2, 3]. According to the statistics, the power consumption of tunnel lighting accounts for more than 50% of the total energy consumption for a tunnel [4, 5, 6]. In China, there were 21,316 road tunnels with a total length of 21,999 km by the end of 2020, and the total length of road tunnels still increases at a rate of more than 10% each year [7, 8]. With the continuous growth of total road tunnel length, tunnel lighting faces a serious problem of energy consumption [9, 10]. In recent years, LED lamps have become the first choice for tunnel energy-saving lighting [11, 12, 13]. Fruitful studies have been performed covering the energy-saving effects and layout optimization of LED lamps in tunnels. [PERSON] and [PERSON] [14] and [PERSON] et al. [15] investigated the influence of lamp layout on lighting energy saving in road tunnels for the symmetric lighting, counter-beam lighting and pro-beam lighting situations. [PERSON] and [PERSON] [16] established a parameter optimization model to acquire the most energy-saving luminaire installation parameters including luminaire installation height and angle,longitudinal and transverse installation spacing, and luminaire power in the interior zone of road tunnels. [PERSON] et al. [17] analyzed the road surface illumination and evenness obtained from different lighting simulations based on DIALux software to get the best scheme for tunnel lamp installation. [PERSON] et al. [18] revealed the influence of the lighting system on the driver's behavior in road tunnels by means of driving simulator experiments. [PERSON] et al. [19], [PERSON] et al. [20] and [PERSON] et al. [21] studied the effects of the spatial distribution of lighting environment parameters on drivers' visual performance. [PERSON] et al. [22] proposed an optimal design model for determining light sources, powers, layout modes and equipment spacing of lamps in tunnels, which can not only effectively reduce the overall installation and operation cost, but also ease the complexity of manual calculations during the design process of tunnel lighting systems. The use of LED lighting can save energy consumption on the basis of ensuring the tunnel lighting requirements, and the existing statistical data show that the application of LED lamps in the tunnel can achieve energy saving of more than 30% compared with other types of illumination lamps [23; 24; 25]. For the LED lighting environment, the lighting specifications of the \"_Guide for the Lighting of Road Tunnels and Underpasses_ (CIE 88 2004)\" [26] and \"_Guidelines for Design of Lighting of Highway Tunnels_ (JTG/T D70/2-01-2014)\" [27] specify the road surface luminance in tunnels and demand that the average luminance of the tunnel sidewalls, up to at least a height of 2 m, must be at least 60% of the average road surface luminance at the relevant location. In the existing tunnel lighting design, the lighting calculation and analysis focus on the setting of road surface luminance. As for the sidewall luminance, it simply follows the principle of no less than 60% of the road luminance. In fact, the tunnel sidewall is one of the important components in tunnel lighting environment; however, the impact of the improvement of sidewall brightness on the LED lighting environment in the tunnel has not attracted enough attention. For example, when the sidewall luminance is 80% of the road surface luminance, or even greater than the road luminance, what impact will it have on the lighting environment in the tunnel and the visual characteristics of human eyes?