Investigation on Mixed Reflection Behavior of Cool Pavement Coating and Its Impact on Safety of Road Light Environment

Engineering ›› DOI: 10.1016/j.eng.2025.06.014

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Hui Li ^a^,^b^,^*
, Ning Xie ^a^,^c
, Xue Zhang ^a
, Lijun Sun ^a^,^*
, John T. Harvey ^d
, Lei Wang ^a

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Abstract

Due to low reflectance (5%–10%) of black asphalt pavement, most of the incident solar radiation (295–2500 nm) is absorbed and stored. This results in the high temperature (even exceeding 70 °C in summer) of the pavement surface, which leads to pavement diseases, exacerbates the heat island effect, and reduces human thermal comfort. Reflective pavement coating with high reflectance ranging from 20% to 80% is an effective way to solve the above problems. However, excessively improving the visible reflectance and ignoring the mixed reflection behavior (including specular and diffuse reflection properties) may cause glare problems and negatively affect road light environment safety. Therefore, precise control of reflectance is very significant. In this study, an automated test platform of reflection behavior was developed to investigate the mixed light reflection distribution pattern of reflective coating. Additionally, the impact of reflective coating on the light environment was explored. It was found that there was obvious specular reflection under conditions of a low incidence angle (less than 10°). Moreover, reflective coating could change the lightness index and specular reflection coefficient of traditional pavement. Finally, considering glare during the daytime and the nighttime illumination safety, the control indexes of reflective pavement coating were proposed. The visible reflectance and specular reflection coefficient should be lower than 22% and 1.5, respectively. The results will provide a theoretical basis for the precise and safe design of reflective pavement coatings to improve driving safety as well as the pavement light and thermal environment.

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Keywords

Road infrastructure / Pavement engineering / Cool pavement / Reflective coating / Optical characteristic / Glare safety / Light environment

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Hui Li, Ning Xie, Xue Zhang, Lijun Sun, John T. Harvey, Lei Wang. Investigation on Mixed Reflection Behavior of Cool Pavement Coating and Its Impact on Safety of Road Light Environment. Engineering DOI:10.1016/j.eng.2025.06.014

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1. Introduction

In recent years, the continuous urbanization across the world has caused significant environmental impacts, especially the urban heat island (UHI) effect [1,2]. According to the latest 2018 Revision of World Urbanization Prospects Report, by 2050, the proportion of the world population living in urban cities will increase to 9.8 billion [3], causing more severe ecological effects. As significant transportation infrastructure, pavements account for 30%–40% of the areas in typical cities around the world [4]. Among them, due to advantages such as low construction and maintenance costs, driving comfort, and low noise, asphalt pavement occupies a large proportion of road paving. Black asphalt pavement has a very low reflectance of solar radiation (5%–10%), most of the solar radiation incident on the pavement is absorbed and stored, resulting in road surface temperatures in summer exceeding 70 ℃ [5]. Due to the strong temperature sensitivity of asphalt pavement, high-temperature diseases such as instability, rutting, and cracking intensify, which seriously endangers structural resilience for safety and infrastructure service life [6]. In addition, heat stored in road structure is released into the environment, exacerbating the UHI effect, reducing human thermal comfort, and even threatening life safety [7]. Under the trend of global warming and extreme heat events [8], the cities are facing tougher high-temperature challenges.

Research scholars have verified the effectiveness of increasing pavement reflectance in reducing temperature [9,10]. In 2008, the United States Environmental Protection Agency (USEPA) defined cool pavement for the first time as a technology and method to mitigate the UHI effect, requiring it to reflect more solar radiation or otherwise to achieve cooling [11]. Generally, cool pavement technology includes reflective pavement coating [12], permeable pavement [13], high-conductive pavement [11], and so forth. Among these, reflective pavement coatings with superior construction convenience can decrease pavement surface temperature by 3–15 ℃ [11,12,14,15]. In view of this, it has already been recognized as one of the most effective mitigation measures against the UHI effect, especially in drought conditions, and is more affordable in terms of maintenance costs [16]. Several studies [[17], [18], [19], [20]] have explored the cooling performance of cool pavement coatings, developed new materials, and demonstrated their effectiveness in reducing temperatures. Los Angeles and Phoenix in the United States both have tried to apply cool pavement coatings in 2019 and 2021, achieving temperature reductions of 5 and 11 ℃, respectively [19,20].

Overall, the exploration of reflective coating technology for pavement has been carried out worldwide, mainly focusing on optimizing cool pavement coatings based on their cooling effect and maximized reflectance [14]. However, it is worth noting that solar radiation is distributed in the ultraviolet, visible, and near-infrared light bands, and increasing the reflectance of different light bands of the pavement can have varying effects on the optical and thermal environment [21]. Excessively increasing the visible reflectance may increase the risk of glare and pose a threat to visual safety [11,16,22,23]. In Japan, 136 km of white reflective pavement has been paved since 2017 to ensure the thermal comfort of marathon runners for the Summer Olympics. It was found that the “glare” phenomenon of white reflective pavement was more pronounced compared to permeable pavement in practical investigations [24]. In recent years, scholars have begun to develop colored cool pavement coatings to avoid glare problems caused by white coatings [17,21,[25], [26], [27]]. Xie et al. [28] explored the full-spectrum optical properties of cool pavement coatings and found a linear relationship between lightness and reflectance. However, the mixed reflection behavior of cool pavement coatings, including the specular and diffuse reflection, also influences the daytime and nighttime light environment. There is a lack of research on the regularities and effective control parameters.

In order to investigate the risk of glare caused by reflective road surfaces, it is necessary to first explore their reflective distribution characteristics. In fact, pavement surfaces are not purely diffuse reflectors but exhibit a mixed reflection pattern of diffuse and specular reflection. It has different reflection distribution responses to varying incident angles, which significantly impact the road lighting environment [26]. Changes in the reflective properties of the pavement surface after applying a coating can further affect the visual safety under both daytime and nighttime lighting conditions. Nevertheless, most of the current studies treat reflective pavement materials as ideal diffuse reflectors and ignore specular reflection when investigating their optical properties [29]. Therefore, the mixed reflection pattern has better to be explored, based on the full-spectrum characteristics of reflective coatings, to better understand its impact on the light environment.

There is a limited number of exploratory studies on the mixed reflection characteristics of reflective pavement coatings, but several studies in other fields can be referenced. For example, Carnielo et al. [30] investigated the mixed reflection properties of non-transparent building materials and their impact on building energy consumption. In 2015, Rossi et al. [31] developed a retro-reflective material and investigated its reflective properties at different incident angles. They found that the coating exhibited retroreflective properties when the incident angle was small. In 2016, Zinzi et al. [32] analyzed building materials (granite, asphalt shingles, polyvinylchlorid film, and polyolefin film) using the bidirectional reflectance distribution function (BRDF). The angular dependence of these materials’ reflective properties was explored, verifying their mixed reflective properties. In 2018, Rossi et al. [33] investigated the relationship between reflective properties and incident angles of photocatalytic coatings using the BRDF. In 2022, Galatanu and Canale [34] developed a field test method based on a portable charge coupled device (CCD) camera to obtain BRDF parameters of asphalt pavements. Their findings revealed that the diffuse reflection phenomenon was more pronounced when the incident angle was large. Muzet et al. [35] developed a portable tester for luminance coefficient (COLUROUTE) to obtain the reflective properties of asphalt pavement and provide luminance coefficient tables as specified by the International Commission on Illumination (Commission Internationale de ĺEclairage, CIE).

How can the impact on the light environment be evaluated? The light environment is mainly divided into two aspects: the daytime and nighttime light environments, with sunlight and artificial illumination serving as the respective light sources, respectively. For glare assessment caused by reflections under bright vision conditions during daylight, previous studies mainly focused on the glare caused by photovoltaic (PV) panels. In recent years, with the increasingly widespread application of PV panels in buildings and airports, their reflected glare has also attracted academic attention [36]. Notably, the reflectance of PV panels is approximately 5% at low incident angles, but when the incident angle increases to 45°, the reflectance can reach up to 15%. A series of studies has sought to reveal the glare mechanism and deal with the problem of visual safety risk. Babin et al. [37] analyzed the daytime reflected glare of PV panels and compared the BRDF of structured glass with antireflective (AR) coatings. They found that antireflective coatings were not sufficient to eliminate glare risks, such as flash blindness. Sandia National Laboratories operated by a contractor of the U.S. Department of Energy’s National Nuclear Security Administration developed a tool named SGHAT to assess glare from PV panels based on the glare theory of Ho et al. [38]. In their study, the degree of glare was categorized into three stages according to retinal irradiance: low potential for after-image, potential for after-image, and permanent eye damage (retinal burn) [39]. Although existing research on direct daylight glare and reflective glare from PV panels has not directly provided an analysis method for the glare threshold or daytime reflective glare, it has offered valuable insights and references for evaluating the potential risk of glare from reflective pavement coatings in this study.

In terms of nighttime light environment, a few studies on the impact of pavement surface characteristics on the lighting environment were conducted by experts in the field of illumination in the 1970s. One of the most representative achievements was the road lighting design method based on the simplified luminance coefficient (r)-tables proposed by the CIE [40], which provided the average luminance coefficient Q₀ and specular reflection coefficient S₁. Jackett and Frith [41] conducted a statistical analysis of the reflective properties of road surfaces in New Zealand. Their result indicated that the reflective properties of current roads (Q₀ = 0.05, S₁ = 0.57) were significantly lower than the designed illumination values (Q₀ = 0.09, S₁ = 0.58), negatively affecting drivers’ ability to detect obstacles. Therefore, it is of great significance to investigate the optical behavior of pavement coatings, including specular and diffuse reflection. Based on the mixed reflection properties, control indices oriented towards visual safety and application scenarios for effectively mitigating the heat-island effect can be proposed, contributing to a more comfortable and safer driving environment.

The objectives of this study are to ① investigate the mixed reflection behavior and distribution pattern of reflective pavement coating materials using an independently developed mixed reflection test platform; ② analyze the influence of reflective pavement coatings on daytime lightness and nighttime illumination parameters based on their mixed reflection behavior, considering daylight reflection glare and nighttime lighting conditions; and ③ propose effective control indicators and threshold ranges for reflective pavement coatings with a focus on ensuring light environment safety during both daytime and nighttime. The results will provide a theoretical basis for the precise and safe design of reflective pavement coatings, contributing to improved road safety and the alleviation of the UHI effect.

2. Materials and methodology

2.1. Materials

Twelve reflective cooling asphalt concrete specimens with four types of coatings (white, red, gray, and black) and three different void ratios (7%, 17%, and 24%) were prepared for the experiments. All coatings were composed of water-based acrylic emulsion, functional fillers and pigments, additives, and deionized water. The TiO₂ was selected as the reflective pigment, while red iron oxide and black CuO were used as colored pigments. For white coatings, the functional fillers included conventional rutile titanium dioxide (R-TiO₂) and near-infrared titanium dioxide (NIR-TiO₂) pigments. For colored reflective coatings, considering better optical performance in the near-infrared solar radiation region, the near-infrared TiO₂ was selected as the functional reflective filler. Regarding the components of the colored coatings, the proportions of red iron oxide, R-TiO₂, and NIR-TiO₂ were 5:25:8, which were determined in Ref. [27]. Additionally, for all coatings used in this study, ZnO with a particle size of 30 nm was selected as the anti-aging additive (results of the preliminary research in Ref. [3]). The preparation process for the samples was detailed in Ref. [28]. The composition of the four reflective cooling coatings is shown in Table 1, and the basic information and reflectance properties of the 12 reflective cooling asphalt concrete specimens are presented in Fig. 1 and Table 2.

Asphalt concrete specimens were prepared as substrate materials for reflective coatings by using performance grade (PG)-88 high-viscosity asphalt, basalt as aggregates, and mineral powder as filler. The design gradation of asphalt mixture specimens with target void ratios of 7%, 17%, and 24% is shown in Table S1 in Appendix A.

2.2. Methods

2.2.1. Models used for mixed reflection theory

The surface of an ideal diffuse reflector responds uniformly to incident light from any direction and remains unchanged at all viewing angles. Its radiation intensity distribution obeys Lambert’s cosine law. Unlike diffuse reflection, the optical properties of specular reflection depend on incident light angle and the viewing angle, following the law of reflection. Most existing studies assume that asphalt pavement and reflective coating materials are ideal diffuse reflectors. However, when considering their mixed reflection characteristics, which include both specular and diffuse reflection, the reflection distribution is shown in Fig. 2.

The BRDF is used to characterize both specular and diffuse reflections. It is defined as the differential ratio of the reflected luminance at a certain angle to the incident illuminance at another angle. First proposed by Nicodemus [42] in 1965, BRDF can be measured using two methods: a goniophotometer platform [34] and portable test equipment employing the integral hemisphere method [43].

To describe the mixed reflection characteristics of the road surface, which include both diffuse and specular reflections, a model for characterizing the mixed reflection characteristics of the road surface under light source incidence is established based on the above BRDF model, as shown in Eq. (1) [40]. The road surface luminance coefficient q(α, β, γ) is the BRDF used for road light environment analysis. It is defined as the ratio of the road surface lightness to its illumination at a fixed angle. The angle definition used for characterizing the mixed reflection behavior of the road surface is shown in Fig. 3 [40].

(1)

B R D F = q (α, β, γ) = \frac{L}{E}

Where α represents the observation angle in the horizontal direction,

β

is the angle between the incident surface of the light source and the observation surface,

γ

is the incident angle of the light source in the vertical direction, L is the luminance of a certain point on the road surface, and E is the horizontal illuminance.

Point P and the surface luminance L observed at the observer’s position can be calculated using the luminance coefficient q and illuminance E, as shown in Eq. (2). To simplify the calculation, the CIE introduced a simplified luminance coefficient r [43], which is expressed in Eq. (3). Considering that the driver’s typical viewing angle range on the road is generally 60–160 m ahead, with an average eye height of 1.45 m, the α-angle range can be approximated as 0.5°–1.5°. To standardize the calculation, α is taken as 1° in the road luminance model. In this case, the simplified luminance coefficient r becomes a function of β and γ only. The simplified luminance can be calculated by Eq. (4). The ranges of β and γ are 0°–180° and 0°–90°, respectively.

(2)

L = q (α, β, γ) \cdot E = q (α, β, γ) \cdot \frac{I (c, γ)}{h^{2}} \cdot c o s^{3} (γ)

(3)

r (α, β, γ) = q (α, β, γ) \cdot c o s^{3} (γ)

(4)

L = \frac{I (c, γ)}{h^{2}} r (1, β, γ)

where

I (c, γ)

is the spatial distribution function of light intensity of a light source, which generally refers to the luminous intensity in the

c

and γ directions.

From the driver’s field of vision, the luminance of each point on the road varies, so it is necessary to consider the average value within a certain three-dimensional range. Within the spatial region Ω₀, the average luminance coefficient

Q_{0}

and specular reflection coefficients

S_{1} a n d S_{2}

can be calculated according to Eqs. (5), (6), representing the brightness and specular reflection level of the road surface. Based on the actual road conditions, the spatial constraint region Ω₀ is illustrated in Fig. 4. Due to the existence of specular reflection and diffuse reflection, the CIE categorizes roads into four classifications, R1–R4, as shown in Table 3 [40]. Among them, each classification corresponds to a standard simplified luminance coefficient r-table, which can be referenced in CIE 144: 2001 [40] In this study, the simplified luminance coefficient r-tables will be measured by the mixed reflection test platform, which will be described in Section 2.2.3.

(5)

Q_{0} = \frac{\int q (α, β, γ) d Ω_{0}}{\int d Ω_{0}}

(6)

S_{1} = \frac{r (β = 0^{°}, \tan (63^{°}) = 2)}{r (β = 0^{°}, \tan (0^{°}) = 0)}, S_{2} = \frac{Q_{0}}{r (0, 0)}

2.2.2. Mixed reflection test platform and test method

To investigate the mixed reflection behavior of reflective coating materials, an automated mixed reflection test platform was independently designed and constructed for this study. A detailed description of this test platform can be found in Appendix A. Based on this platform, fully automated measurements of the pavement materials’ luminance under the three key angles α, β, and γ can be performed.

After obtaining the luminance of different points using the automated test platform, there are three points to consider when processing data. The first one is the simplified luminance coefficient r-table. The second one is how to transform the r distribution from the spherical coordinate system into the rectangular coordinate system (x, y) to illustrate the specular and diffuse reflection distributions. The third consideration is the calculation of the average luminance coefficient Q₀ and the specular reflection coefficient S₁ for a certain spatial range. The calculation and analysis methods are as follows:

(1) After measuring 580 (29 × 20) data points with the test platform, the coefficient r under different angle conditions can be calculated using Eqs. (7), (8).

(7)

Ω_{0} = \int_{0}^{Ω_{0}} d w = \int_{0}^{Ω_{0}} s i n γ d γ d β = \int_{0}^{180} \int_{0}^{u p p e r l i m i t} s i n γ d γ d β

(8)

Q_{0} = \int_{0}^{Ω_{0}} q d w = \int_{0}^{180} \int_{0}^{u p p e r l i m i t} r t a n γ d (t a n γ) d β

(2) The actual light source incidence/observation angle is regarded as the incident/observation angle under spherical coordinates after obtaining the simplified luminance coefficient distribution map of the luminance coefficient under the angles of γ (0° < γ < 85°) and β (0° < β < 180°). This is transformed into the reflection distribution in the rectangular coordinate system to more clearly characterize the mixed reflection where diffuse reflection and specular reflection coexist.

(3) As for the calculation process of Q₀ and S₁, considering that the test system provides discrete measurements, the continuous Q₀ value can be obtained by stereo interpolation and numerical integration as shown in Eqs. (7), (8) (

w

is the solid angle). In this paper, MATLAB programming was used to get the coefficient Q₀ by uniformly interpolating the r-values and then integrating them. The specular factor S₁ is calculated using Eq. (9).

(9)

S_{1} = \frac{r (0, 2)}{r (0, 0)}

2.2.3. Light environment impact assessment

In this study, the effects of the light environment are primarily focused on the sunlight reflection glare during the daytime and the lightness, uniformity, and glare at night. The threshold indicators and impact assessment methods are illustrated as follow.

(1) Daytime light impacts. Considering the saturation effect caused by the high reflectance of the pavements [44], the average luminance L_av was chosen as an evaluation index for the saturation effect glare produced by reflective pavements under daylight conditions. The average luminance L_av in cd·m⁻² is determined by the luminance coefficient of the road surface and the normal vertical illuminance E_s of sunlight. The luminance coefficients of the road surface at different angles are characterized using the BRDF. For the normal vertical illuminance of the sunlight, the precise relative position of the sun at a specific point on the ground plane is determined by the azimuth angle and the altitude angle. Therefore, variations in the relative position of the sun result in changes to its illuminance and incident angle throughout the day and across the year. In this study, Shanghai (120.86°E, 30.75°N) in China was used as the standard location to obtain the solar altitude angles for the entire day at the four key time nodes: the Spring Equinox, Summer Solstice, Autumn Equinox, and Winter Solstice. The empirical formulas of the sunlight’s normal vertical illuminance E_s at different altitude angles were considered. The calculation formula for received luminance as perceived by human eyes can be corrected using Eqs. (10), (11).

(10)

L_{a v} = q (BRDF) \frac{E_{s}}{π}

(11)

E_{s} = 1.37 \times 1 0^{5} e^{- \frac{0.223}{\sin h_{0}}}

Where

q (BRDF)

is the luminance coefficient in the BRDF of the pavement material, which is derived through the mixed reflection test platform;

E_{s}

is the direct solar normal illuminance in lux; and

h_{0}

is the solar elevation angle (°).

There is no uniform standard for the control threshold index of luminance. German researchers proposed a luminance acceptance threshold of 1.0 × 10⁴ to 1.6 × 10⁵ cd·m⁻² for the human eye [44]. Beyond this threshold, the human eye’s adaptability fails to function effectively, causing absolute glare and potentially leading to flash blindness. In the architectural field, road glare generated by glass curtain walls is categorized according to the acceptable luminance of the human eye at different incident angles of sunlight (< 15°, 15°–30°, and > 30°). Therefore, this study combines the aforementioned luminance thresholds and refines them into the threshold table shown in Table 4. It is worth noting that, due to the driver’s age affecting his visual acceptance, this study only considers the visual characteristics of drivers aged 25–30, as recommended by the CIE for analysis. To account for the most unfavorable conditions, it was assumed that the driver is directly facing the sun and that the incidence angle of sunlight is equal to the solar elevation angle at that moment. Based on the above assumptions, change law curves of the received luminance by human eyes under different seasons, times of day, and reflective properties of road surface materials were drawn.

(2) Nighttime light impacts. Under nighttime conditions with artificial street lighting, the reflective optical properties of the road surface not only influence the average lightness of the road environment in a given area but also significantly impact overall uniformity and longitudinal uniformity. The average luminance

L_{a v}

as well as the overall uniformity

U_{0}

and the longitudinal uniformity

U_{L}

were considered in this study. The indicators Q₀ and S₁ are partial data that represent the characteristics of material samples and serve as the most direct parameters for illustrating mixed reflection regularities. In contrast,

L_{a v}

U_{0}

, and

U_{L}

are parameters characterizing pavement performance in real-world scenarios. These indicators can be calculated using Eqs. (12), (13), respectively.

(12)

U_{0} = \frac{L_{m i n}}{L_{a v}}

(13)

U_{L} = \frac{L_{m i n}}{L_{m a x}}

where

L_{m i n}

and

L_{m a x}

represent the minimum and maximum luminance of the pavement as observed from the observer’s position and parallel to the road axis (cd·m⁻²).

According to the CIE 140: 2019 Road Lighting Calculations (2nd Ed) [45], the glare threshold increment (TI, %) should be calculated as shown in Eq. (14) for the pavement luminance in the range of 0.05 < L_av < 5 cd·m⁻². Since the luminance of the pavement increases significantly after applying heat-reflective pavement coatings compared to the original pavement, this study also considers the condition L_av ≥ 5 cd·m⁻², in addition to the range of 0.05 < L_av < 5 cd·m⁻². It can be calculated using Eq. (15) with reference to the CIE 115: 2010 Lighting of Roads for Motor and Pedestrian Traffic (2nd Ed) [46].

(14)

T I = 65 \frac{L_{v}}{{L_{a v}}^{0.8}}, 0.05 < L_{a v} < 5

(15)

T I = 95 \frac{L_{v}}{{L_{a v}}^{0.8}}, L_{a v} \geq 5

Where

L_{v}

is the veiling luminance in cd·m⁻².

The latest version of DIALUX 4.13 (DIAL GmbH company, Germany) was selected to simulate the roadway lighting environment. The BRDF data was imported to calculate and acquire the parameters required for the simulation model. Two-lane road models were used, with each lane having a width of 3.5 m, which is a common standard in practical engineering. The central strip between the two directions was 1.5 m wide and 0.1 m high. NVC NRS0006/250W (NVC Lighting, China) high-pressure sodium (HPS) road lights were chosen as the lighting source for the simulation. According to general installation conditions in practice and the design requirements specified in CJJ45–2015 [47], the installation height was set to 10 m, and the distance between two light poles was 30 m. The cantilever length of the lights was set to 1.5 m, and the dip angle was set to 10.0°. The detailed lighting requirements are listed below [47]:

(1) The installation height H ≥ 0.7W_eff, where W_eff is the effective width of pavement;

(2) The pole spacing S ≤ 3H;

(3) The cantilever should be less than 1/4H, and the dip angle should not exceed 15°.

Three basic conventional lighting arrangements were considered: a bi-directional symmetrical arrangement, a bi-directional staggered arrangement, and a symmetrical arrangement in the median. Each arrangement incorporated the reflectance characteristics of pavements as input parameters. Four virtual observers were placed in each lane to simulate virtual drivers, located 1.75 m from the edge of each lane. Illumination and observer locations are shown schematically in Fig. 5.

3. Results and discussion

3.1. Dependence of the reflection properties of asphalt pavement on angles

According to the variation of the simplified luminance coefficient r with the incident angles β and γ, three-dimensional (3D) and two- dimensional (2D) contour plots were generated as shown in Fig. 6. It is evident that the reflective properties of the original asphalt pavement material under dry conditions depend on both incident angles β and γ. For the asphalt concrete pavement R1, which behaves similarly to an ideal diffuse reflector, the simplified luminance coefficient r gradually decreases as the incident angle γ increases, while remaining nearly independent of the angle β (the angle between the incident plane and the vertical plane). In contrast, for the mixed reflective pavements R2, R3, and R4, which exhibit both specular and diffuse reflection, their simplified luminance coefficients r tends to increase initially and then decrease as the incident angle γ increases.

Based on the visualization of the angular dependence of pavement reflection characteristics through the three-dimensional contour map, the following pattern emerges: ① The spike represents the change of the simplified luminance coefficient r with the incident angle γ; ② the torsion illustrates how the simplified luminance coefficient r changes with the angular variations of β and γ. For pavement R1, the reflection characteristics are minimally affected by the β and γ angles, making it close to a completely diffuse reflector. For pavements R2–R4, the spikes and torsion in the contour plots gradually increase, indicating a higher degree of specular reflection.

Under different incidence angles, as β and γ vary, the simplified luminance coefficient patterns of the original asphalt concrete pavement with three different void ratios are shown in Fig. 7. It is observed that as the incident angle γ increases, the simplified luminance coefficients r all exhibit a pattern of initially increasing and then decreasing, which is consistent with the behavior of standard pavements R2–R4. Furthermore, the original asphalt pavement material with a void ratio of 7% demonstrates a significantly higher degree of specular reflection and greater sensitivity to the incidence angle compared to the other pavement samples with void ratios of 17% and 24%.

To investigate the variations in the average luminance coefficient Q₀ of the original asphalt pavement material in the dry state as a function of the observation angle, this study fixed the incidence angles β and γ as 0° and selected eight different observation angles α: 1°, 2°, 5°, 10°, 20°, 30°, 45°, and 60°. The average luminance coefficient Q₀ of the original asphalt pavement material in the dry state and the specular reflectance factor S₁ are presented in Fig. 8. The results demonstrate that with the increase in the observation angle, the average luminance coefficient Q₀ and the specular reflection coefficient S₁ both exhibit a decreasing trend. The average luminance coefficient Q₀ decreases significantly at smaller observation angles, while at larger observation angles, the decreasing trend gradually converges to a straight line. The result suggests that the material’s behavior becomes more diffuse-like as the observation angle increases. For pavement materials with different void ratios, the findings revealed that the critical points for transitioning from mixed reflection to diffuse reflection vary. For asphalt concrete pavements of type R1, the critical point occurs at an observation angle of approximately 20°, while for R2 and R3, the observation angle corresponding to the critical point is approximately 10°.

As the observation angle increases, both the average luminance coefficient Q₀ and the specular reflection coefficient S₁ of the road surface decrease. The change in Q₀ is gradual, while the decrease in the specular reflection coefficient S₁ is more pronounced. The specular reflection coefficient values S₁ = 0.42, 0.85, and 1.35 are labeled in Fig. 9 in accordance with CIE’s road grading standards R1–R4, with these values serving as dividing lines. It can be found that pavement with different porosities performs differently. For dense asphalt concrete pavement with a 7% void ratio, it exhibits strong specular reflection at an observation angle of 1°. With a further increase in the observation angle, the degree of specular reflection gradually decreases and transitions to slight specular reflection. For asphalt concrete with a 17% void ratio, it consistently exhibits mild specular reflection. For asphalt concrete with a 24% void ratio, it initially shows mild specular reflection at observation angles of 1° and 2°, before transitioning to mixed reflection dominated by diffuse reflection.

3.2. Mixed reflection behavior of reflective coatings under dry conditions

After applying different types of coating materials onto asphalt concrete surfaces with different void ratios, the mixed reflection patterns of the reflective pavement cooling coating materials were analyzed. The characteristic photometric body under the right-angled coordinate system obtained from the r-table is shown in Fig. 10. The total area of the characteristic photometric body represents the average luminance coefficient, and the degree of deviation from the sphere represents the degree of specular reflection. The horizontal and vertical coordinates of each column in the plot are kept the same, with the red plot line representing the angle of incidence γ, ranging from 0° to 90°, and the blue plot line representing the azimuth β, ranging from 0° to 180°. To better demonstrate the specular reflection, the range of the horizontal and vertical coordinates of each column is kept basically consistent, but no uniform provisions are made for the range of the horizontal and vertical coordinates of each row.

Further calculations of the average luminance coefficient Q₀ and specular reflection coefficient S₁ are shown in Fig. 11. The results indicate that the reflection pattern of reflective pavement coating on asphalt concrete surfaces manifests as mixed reflection, where diffuse reflection and specular reflection coexist. Under dry conditions, when coated with reflective materials, the surface average luminance coefficient Q₀ increases, while the specular reflection coefficient S₁ exhibits different trends. Compared with the original asphalt pavement, the average luminance coefficient Q₀ increases by 4–5 times for white coatings, 2–3 times for red and gray coatings, and approximately 1.2 times for black coatings. Meanwhile, the specular reflectance factor S₁ for the white coating is only 20%–30% of the original asphalt concrete, for the red coating, it is 45%–70%, and for the gray coating, it is 62%–87%. Conversely, the specular reflection coefficient S₁ for the black coating significantly increases to 1.57–1.75 times. The result indicates that due to TiO₂ and other reflective materials, the increased reflectance improves the lightness of the road surface and reduces the gloss of the asphalt concrete surface, thereby controlling surface specular reflection. Because of the low content of TiO₂ in black coating, the surface gloss increases, and the specular reflection is enhanced.

This study analyzed the effect of materials on mixed reflection behavior. The filler content of titanium dioxide P(TiO₂), pigment volume concentration (PVC), the visible light reflectance (R_vis), and the total reflectance (R_total) were chosen as the parameters, while the average luminance coefficient Q₀ and the specular reflection coefficient S₁ were chosen as the reflection parameters. The correlation matrix is shown in Fig. 12. The results indicate that the higher the TiO₂ content, the greater the average luminance and the lower the degree of specular reflection. This is because the TiO₂ not only acts as a reflective material but also increases the roughness of the micro-surface, thereby further reducing the degree of specular reflection. Both visible light reflectance and total reflectance primarily influence the average luminance coefficient. The higher the total reflectance, the greater the luminance, however, its effect on the degree of specular reflection is not significant. In addition, due to the flaky nature of copper oxide (CuO), the degree of specular reflection on its surface significantly exceeds that of the original asphalt concrete specimen.

3.3. Mixed reflection behavior of reflective coatings under wet conditions

The original asphalt pavement coated with reflective coatings was initially saturated with water. Then the mixed reflection pattern of the surface was then tested at 0, 15, 30, and 60 min. The average luminance coefficient and specular reflection coefficient were calculated as shown in Fig. 13. Consistent with the behavior of original asphalt pavement, the average luminance coefficient of the reflective coating under wet conditions decreases, while the specular reflection coefficient increases. However, compared with uncoated asphalt concrete, the specular reflection of the white, red, and gray coatings remains low, and most of these pavements still belong to the R1, R2, and R3 types. Due to the high specular reflection coefficient of black coating, it is still classified as the R4 strong specular reflection pavement. When the surface is first wetted, the presence of the water film causes the specular reflectance factor of all coating types to increase by approximately 1.0 to 1.5 times compared to the dry state. Within 60 min after wetting, the specular reflectance factor of reflective asphalt pavement with different void ratios decreases significantly, eventually approaching the dry state, as shown in Fig. 14. The results clearly show that after applying the reflective coating, the road surface lightness remains higher, and the degree of specular reflection increases under wet conditions.

3.4. Impact analysis of daytime glare based on saturation effect

According to the daytime glare safety evaluation index, this study obtained the solar altitude angle data of four solar terms in Shanghai of China in 2022, namely, the Spring Equinox (March 20), Summer Solstice (June 21), Autumn Equinox (September 23), and Winter Solstice (December 22). Then the lightness difference between different types of reflective coatings and the original asphalt pavement under daylight conditions was further analyzed. Since the void ratio has a limited effect on surface lightness and glare lightness exhibits a similar pattern, this paper focuses on the lightness changes of pavement with a 7% porosity after applying different types of coatings. As shown in Fig. 15, the lightness of pavements coated with reflective coatings is significantly enhanced. This leads to an increased risk of discomfort glare from sunlight reflection, but absolute glare (visual blindness) was not observed. For white-coated pavement, there is a clear area of strong influence, while for other types of pavements, the uncomfortable glare caused by daytime reflection is slight and mainly occurs in the early morning and at sunset. For ordinary asphalt concrete, there is no glare effect due to its low luminance coefficient at all incidence angles. In different seasons, owing to the variation in solar altitude angle, it was observed that although the sunshine duration is shorter on the Winter Solstice, the glare effect is more noticeable. This occurs because the sun’s altitude angle (angle of incidence) on the Winter Solstice ranges widely from 0° to 30°.

To further analyze the daytime glare impact of reflective pavement cooling coating materials, the range of “glare time window” was calculated based on the lightness change curve shown in Fig. 15. The results of the calculations of the duration influence difference for impacts are presented in Fig. 16. All types of asphalt concrete pavements do not exhibit any glare impact on a typical day. However, after applying reflective coatings, the duration of uncomfortable glare ‘strong impact’ for white-coated pavement on typical days ranges from 100 to 160 min, with the glare impact transitioning from ‘no impact’ to ‘slight impact’ and eventually to ‘strong impact’. The longest glare time window occurs for the white coating with a 7% void ratio on the Winter Solstice. The glare ‘slight impact’ time window for other types of coated pavements increases significantly, while the ‘strong impact’ time window does not change significantly. For red and gray coatings, the uncomfortable glare (‘strong impact’) duration is within 30 min on a typical day, but the ‘slight impact’ duration increases by 80 to 130 min. For the black coating, similar to conventional asphalt concrete, there is no glare at all on a typical day.

3.5. Impact analysis of nighttime light environment safety

Three roadway lighting modes were established: bidirectional symmetric light distribution, cross light distribution, and center median light distribution. As shown in Fig. S3, the average luminance L_av, overall uniformity U₀, longitudinal uniformity U_L, TI, and the corresponding standard deviation were calculated for these three conditions. It was observed that, compared to the original asphalt pavement, the average luminance L_av of coatings increased significantly, while the overall luminance uniformity decreased, the longitudinal uniformity showed a slight decrease with minimal change, and the TI decreased significantly. When comparing the calculation results across various lighting arrangements, it was found that under the conditions of bidirectional symmetrical lighting arrangement and cross lighting arrangement, the difference in average road brightness was minimal. Although there were some differences in overall and longitudinal uniformity, these differences were less than 10%. The difference in TI was within 3%. In contrast, compared to the two-way arrangement of lights, the central median lighting arrangement resulted in a noticeable increase in average road brightness, accompanied by significant decreases in uniformity and TI. Consequently, the subsequent analysis primarily focuses on the impact of cool coatings on road lighting environmental parameters under bidirectional intersection lighting conditions.

The influence of the average luminance coefficient Q₀ and specular reflection coefficient S₁ of the reflective coatings on light environment indices under nighttime illumination conditions was further analyzed. The correlation matrix was plotted as shown in Fig. 17. The result demonstrates that the average luminance coefficient Q₀ of pavement materials is strongly correlated with pavement luminance, longitudinal uniformity U_L, and TI. As the luminance coefficient Q₀ increases, the pavement luminance exhibits a linear increase, while longitudinal uniformity U_L and TI decrease. There is some correlation between the specular reflection coefficient S₁ and luminance, total uniformity U₀, and TI, but the correlation is weak. As the degree of specular reflection increases, total uniformity U₀ decreases, while the TI increases. Overall, increasing the roadway luminance coefficient improves the overall luminance of the roadway and reduces nighttime glare but negatively affects uniformity in the driving direction.

3.6. Control indicators analysis based on light environment safety

The above analysis reveals that during the daytime, the average luminance coefficient Q₀ of the road surface is closely related to the duration of influence, whereas the relationship between the specular reflection coefficient and the duration of influence is not significant. The relationship between Q₀ and the duration of influence on a typical day is illustrated in Fig. 18. Overall, the duration of strong glare influence increases with higher values of Q₀. When Q₀ < 0.10, there is almost no uncomfortable glare classified as “strong impact.” However, when Q₀ > 0.10, the glare “strong impact” time window increases rapidly. Consequently, to completely avoid the strong impact of uncomfortable glare from daytime reflections, the luminance coefficient Q₀ of the pavement should be maintained below 0.10. Given the strong correlation between the visible light reflectance R_vis and the average luminance coefficient Q₀, it can be concluded that the visible light reflectance should be less than 22%.

To ensure the visual safety of nighttime driving, it is necessary to control the average luminance coefficient Q₀ and specular reflection coefficient S₁ of pavement. The key control indicators include: ① regulating the average luminance coefficient Q₀ through longitudinal uniformity U_L; and ② controlling the specular reflection coefficient S₁ through overall uniformity U₀ and the incremental value of the TI. Consequently, the correlation between the average luminance coefficient Q₀ and longitudinal uniformity U_L was fitted, and regression analysis of the specular reflection coefficient S₁ with overall uniformity U₀ and TI was conducted, shown as Fig. 19. According to the relevant lighting design standard shown in Table S4 in Appendix A, the average luminance coefficient Q₀ and specular reflection coefficient S₁ should be controlled to achieve suitable parameters. Specifically, if longitudinal uniformity U_L needs to be maintained above 0.7, the average luminance coefficient Q₀ should be limited to 0.3 or below. When the TI is required to remain below 10, the specular reflectance factor S₁ should be limited to 1.5 or less to meet the nighttime safety requirements for glare.

In conclusion, the visible light reflectance of reflective coatings should be controlled below 22% to realize the minimum impact of daytime reflected glare, and the specular reflection coefficient S₁ should be restricted to 1.5 or less to ensure the safety of nighttime driving under lighting conditions.

4. Conclusions and recommendations

Sustainable pavement technology involving reflective coating can provide significant ecological benefits in mitigating the UHI effect. However, excessively high reflectance may increase the risk of glare, posing a threat to visual safety on roads. Neglecting the mixed reflection properties of specular and diffuse reflections in reflective coatings may exacerbate this risk and result in negative impacts. The characteristic behaviors, evaluation methods, and control thresholds for road glare safety remain underexplored in current research. This study investigated the mixed reflective behavior of cool pavement reflective coatings using an independently developed test platform. Based on the findings, reflection characteristic control indicators were proposed, considering the daytime reflective glare saturation effect and the safety of nighttime lighting environments under artificial lighting. The main conclusions are as follows:

(1) Raw asphalt concrete pavement and reflective coatings both exhibit a mixed-reflection pattern due to their angular dependence on incident and observation angles. As the observation angle α increases, both the average luminance coefficient Q₀ and the specular reflection coefficient S₁ decrease. The critical point for the transition from mixed reflection to diffuse reflection occurs between 10° and 20°.

(2) After applying reflective coatings, the average luminance coefficient Q₀ of pavement increases due to higher reflectance. The greater the titanium dioxide content, the lower the degree of specular reflection, attributed to the increased roughness of the micro-surface.

(3) The specular reflection coefficient of the asphalt concrete pavement with varying void ratios significantly increases under wet conditions due to water films. The non-specular reflection coefficient decreases significantly within 60 min after drenching and gradually approaches the dry condition.

(4) During the daytime, the lightness of the pavement increases due to reflective coatings, leading to a higher risk of uncomfortable glare from daylight reflection and a longer duration of ‘strong impact’ glare. However, reflective coatings do not cause visual blindness. To completely avoid the strong influence of uncomfortable glare from daytime reflections, the average luminance coefficient Q₀ should be maintained below 0.10, and the visible light reflectance should not exceed 22%.

(5) At night, reflective pavement coatings improve overall roadway lightness compared to original asphalt concrete but reduce lightness uniformity in the driving direction. To control the TI within 10%, the specular reflection coefficient S₁ must be limited to 1.5 or less to satisfy nighttime glare safety requirements.

In summary, the mixed reflection patterns of cool pavement coatings were analyzed, and the control indices for ensuring the safety of the light environment were proposed. These findings provide a theoretical basis for the precise research, development, and safety design of cool pavement coatings. It is recommended that further experimental trials be conducted, focusing on driver human factor tests. Reflective glare thresholds should be established under real roadway conditions to verify the accuracy of glare evaluation methods. Additionally, to ensure driving safety concerning the mechanical performance of cool pavement coatings, it is necessary to further investigate the durability and skid resistance of pavements with heat-reflective coatings. This can be achieved using trial-produced accelerated wear equipment for indoor experiments and outdoor road test sections under varying vehicle loads.

CRediT authorship contribution statement

Hui Li: Supervision, Methodology, Conceptualization. Ning Xie: Writing – original draft, Methodology, Investigation, Formal analysis. Xue Zhang: Writing – review & editing, Investigation, Formal analysis. Lijun Sun: Writing – review & editing. John T. Harvey: Writing – review & editing. Lei Wang: Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to express their gratitude to Mr. Saad Khan from Tongji University for polishing language of the paper. Besides, the research is supported by grants from the National Key Research and Development Program of the Ministry of Science and Technology of the People's Republic of China (2023YFB2604000), the Science and Technology Commission of Shanghai Municipality of China (23210711400), and the Transportation Science and Technology Program of Henan Province (2023-4-2). The sponsorships are gratefully acknowledged. The contents of this paper only reflect the views of the authors and do not reflect the official views or policies of the sponsors.

References

Publishing order | Descend order by publishing year | Descend order by cited within

[1]	Zou Z, Yan C, Yu L, Jiang X, Ding J, Qin L, et al. Impacts of land use/land cover types on interactions between urban heat island effects and heat waves. Build Environ 2021; 204:108138.

[2]	Jato-Espino D. Spatiotemporal statistical analysis of the urban heat island effect in a Mediterranean region. Sustain Cities Soc 2019; 46:101427.

[3]	Xie N, Li H, Zhang H, Zhang X, Jia M. Effects of accelerated weathering on the optical characteristics of reflective coatings for cool pavement. Sol Energy Mater Sol Cells 2020; 215:110698.

[4]	Akbari H, Shea L Rose, Taha H. Analyzing the land cover of an urban environment using high-resolution orthophotos. Landsc Urban Plan 2003; 63(1):1-14.

[5]	Santamouris M, Synnefa A, Karlessi T. Using advanced cool materials in the urban built environment to mitigate heat islands and improve thermal comfort conditions. Sol Energy 2011; 85(12):3085-3102.

[6]	Zhang C, Tan Y, Gao Y, Fu Y, Li J, Li S, et al. Resilience assessment of asphalt pavement rutting under climate change. Transp Res Part D Transp Environ 2022; 109:103395.

[7]	Li H, He Y, Harvey J. Human thermal comfort modeling the impact of different cool pavement strategies. Transp Res Rec 2016; 2575(1):92-102.

[8]	Houghton J. Global warming. Rep Prog Phys 2005; 68(6):1343-1403.

[9]	Santamouris M. Using cool pavements as a mitigation strategy to fight urban heat island—a review of the actual developments. Renew Sustain Energy Rev 2013; 26:224-240.

[10]	Li H, Harvey J, Kendall A. Field measurement of albedo for different land cover materials and effects on thermal performance. Build Environ 2013; 59:536-546.

[11]	Qin Y. A review on the development of cool pavements to mitigate urban heat island effect. Renew Sustain Energy Rev 2015; 52:445-459.

[12]	Santamouris M. Cooling the cities—a review of reflective and green roof mitigation technologies to fight heat island and improve comfort in urban environments. Sol Energy 2014; 103:682-703.

[13]	Li H, Harvey J, Ge Z. Experimental investigation on evaporation rate for enhancing evaporative cooling effect of permeable pavement materials. Constr Build Mater 2014; 65:367-375.

[14]	Sen S, Roesler J, Ruddell B, Middel A. Cool pavement strategies for urban heat island mitigation in suburban Phoenix, Arizona. Sustainability 2019; 11(16):4452.

[15]	Wang C, Wang Z, Kaloush KE, Shacat J. Cool pavements for urban heat island mitigation: a synthetic review. Renew Sustain Energy Rev 2021; 146:111171.

[16]	Pisello AL. State of the art on the development of cool coatings for buildings and cities. Sol Energy 2017; 144:660-680.

[17]	Synnefa A, Santamouris M, Apostolakis K. On the development, optical properties and thermal performance of cool colored coatings for the urban environment. Sol Energy 2007; 81(4):488-497.

[18]	Cao X, Tang B, Zou X, He L. Analysis on the cooling effect of a heat-reflective coating for asphalt pavement. Road Mater Pavement Des 2015; 16(3):716-726.

[19]	Ko J, Schlaerth H, Bruce A, Sanders K, Ban-weiss G. Measuring the impacts of a real-world neighborhood-scale cool pavement deployment on albedo and temperatures in Los Angeles. Environ Res Lett 2022; 17(4):044027.

[20]

Stevens R. Sustainable use of cool pavement and reclaimed asphalt in the city of Phoenix.In: Akhnoukh A, Kaloush K, Elabyad M, Halleman B, Erian N, Enmon II S, et al. editors. Proceedings of the 18th International Road Federation World Meeting & Exhibition, Dubai 2021; 2021 Nov 6–10; Dubai, United Arab Emirates. Cham: Springer International Publishing; 2021. p. 57–71.

[21]	Anting N, Din MFM, Iwao K, Ponraj M, Siang AJLM, Yong LY, et al. Optimizing of near infrared region reflectance of mix-waste tile aggregate as coating material for cool pavement with surface temperature measurement. Energy Build 2018; 158:172-180.

[22]	Rosso F, Pisello AL, Cotana F, Ferrero M. On the thermal and visual pedestrians’ perception about cool natural stones for urban paving: a field survey in summer conditions. Build Environ 2016; 107:198-214.

[23]	Xu L, Wang J, Xiao F, Ei-badawy S, Awed A. Potential strategies to mitigate the heat island impacts of highway pavement on megacities with considerations of energy uses. Appl Energy 2021; 281:116077.

[24]	Zhixiao Ueda, Zhihe Tanaka, Yoshiharu Kamakura, Kenho Narita. Subject’s experiment on glare of thermal barrier pavement. Pavement 2020; 55:19-24. Japanese.

[25]	Coser E, Moritz VF, Krenzinger A, Ferreira CA. Development of paints with infrared radiation reflective properties. Polímeros 2015; 25(3):305-310.

[26]	Levinson R, Berdahl P, Akbari H. Solar spectral optical properties of pigments—part II: survey of common colorants. Sol Energy Mater Sol Cells 2005; 89(4):351-389.

[27]	Xie N, Li H, Zhao W, Zhang C, Yang B, Zhang H, et al. Optical and durability performance of near-infrared reflective coatings for cool pavement: laboratorial investigation. Build Environ 2019; 163:106334.

[28]	Xie N, Li H, Abdelhady A, Harvey J. Laboratorial investigation on optical and thermal properties of cool pavement nano-coatings for urban heat island mitigation. Build Environ 2019; 147:231-240.

[29]	Levinson R, Akbari H, Berdahl P. Measuring solar reflectance—part I: defining a metric that accurately predicts solar heat gain. Sol Energy 2010; 84(9):1717-1744.

[30]	Carnielo E, Zinzi M, Fanchiotti A. On the solar reflectance angular dependence of opaque construction materials and impact on the energy balance of building components. Energy Procedia 2014; 48:1244-1253.

[31]	Rossi F, Castellani B, Presciutti A, Morini E, Filipponi M, Nicolini A, et al. Retroreflective facades for urban heat island mitigation-experimental investigation and energy evaluations. Appl Energy 2015; 145:8-20.

[32]	Zinzi M, Carnielo E, Rossi G. Directional and angular response of construction materials solar properties: characterisation and assessment. Sol Energy 2015; 115:52-67.

[33]	Rossi G, Iacomussi P, Zinzi M. Lighting implications of urban mitigation strategies through cool pavements: energy savings and visual comfort. Climate 2018; 6(2):26.

[34]	Galatanu CD, Canale L. Measuring reduced luminance coefficients for asphalt based on imaging method, IEEE, Prague, Czech Republic. New York City 2022. pp. 1-5.

[35]	Muzet V, Greffier F, Nicola TAїA, Verny P. Evaluation of the performance of an optimized road surface/lighting combination. Light Res Technol 2019; 51(4):576-591.

[36]	Protogeropoulos C, Zachariou A. Photovoltaic module laboratory reflectivity measurements and comparison analysis with other reflecting surfaces, JRC Publication, Valencia, Spain. Brussels 2010. pp. 355-358.

[37]	Babin M, Thorsteinsson S, Jakobsen ML, Spataru SV. Glare potential evaluation of structured PV glass based on gonioreflectometry. IEEE J Photovoltaics 2022; 12(6):1314-1318.

[38]	Ho CK, Sims CA, Yellowhair J, Bush E. Solar glare hazard analysis tool (SGHAT) technical reference manual. Sandia National Laboratories, Albuquerque 2015.

[39]	Ho CK, Sims CA, Christian JM. Evaluation of glare at the Ivanpah solar electric generating system. Energy Procedia 2015; 69:1296-1305.

[40]	Commission Internationale de ĺEclairageCIE 144. Road surface and road marking reflection characteristics. CIE Publication, CIE standard. Vienna 2001, p. 2001.

[41]	Jackett MJ, Frith WJ. Measurement of the reflection properties of road surfaces to improve the safety and sustainability of road lighting. The NZ Transport Agency, Report. Wellington 2009.

[42]	Nicodemus FE. Directional reflectance and emissivity of an opaque surface. Appl Opt 1965; 4(7):767-774.

[43]	Muzet V, Bernasconi J, Iacomussi P, Liandrat S, Greffier F, Blattner P, et al. Review of road surface photometry methods and devices—proposal for new measurement geometries. Light Res Technol 2021; 53(3):213-229.

[44]

Ruesch F, Battaglia M, Brunold S. Methode zur quantifizierung der blendung durch solaranlagen-vergleich mit anderen materialien der gebäudehülle.In: OTTI Symposium Thermische Solarenergie; 2016 Mar 20–22; Kloster Banz, Bad Staffelstein. Regensburg: Ostbayerisches Technologie-Transfer-Institut e.V. (OTTI); 2016. Deutsch.

[45]	Commission Internationale de ĺEclairage. CIE 140: 2019 Road lighting calculations, 2nd ed. CIE standard. Vienna: CIE Publication; 2019.

[46]	Commission Internationale de ĺEclairage. CIE 115: 2010 Lighting of roads for motor and pedestrian traffic, 2nd ed. CIE standard. Vienna: CIE Publication; 2010.

[47]	Ministry of Housing and Urban-Rural Development of People’s Republic of China. CJJ45–2015: Standard for lighting design of urban road. Chinese standard. Beijing: China Architecture Publishing & Media Co., Ltd.; 2015.