# A Methodological Approach to the Study of Retroreflective Pavements
## 3 Results and Discussions
### Stage 3: Effect of the Light Entrance Angle on the Retroreflectance  
Figure 15 illustrates the variation in retroreflectance for the light source entrance angles of 60\({}^{\circ}\), 45\({}^{\circ}\), and 0\({}^{\circ}\) (as shown in Figure 12). The measurements were conducted on the AUTL mixture samples with the extreme colours, white and black, which were manufactured with limestone and porphyry aggregate, respectively. The retroreflectance values for the considered entrance angles are shown in Table 7.  
The results indicate that the retroreflectance values increase in all cases as the entrance angle is raised. This may be attributed to the hollows present on the sample surface, as none of the mixtures considered have a smooth surface. When the incident light is zenithal, the hollows appear darker and do not reflect the light. As the entrance angle increases, along with the observation angle, the darker hollows are perceived as being smaller, thereby reducing the extent of the darkest surface area. This effect could disappear when the light is incident at a very small grazing angle, resulting in a larger reflection area, as shown in Figures 16 and 17. The shade of the mixture has a significant influence. Thus, the retroreflectance of the white AUTL mixture varies from 48.5% at an entrance angle of 0\({}^{\circ}\) to 68.9% at 60\({}^{\circ}\). In contrast, the retroreflectance for the black mixture ranges from 6.7% to 8.4%, with minimal differences due to the low values. Finally, to analyse the effect of texture on these measurements, two additional mixture types were considered, a BBTM (thin mixture) and an AC16S (bittuminous concrete-type mixture). Both types were made using the extreme colours achieved with the porphyry aggregates and 50/70 bitumen (black), as well as the limestone aggregate and synthetic bitumen with titanium oxide (white) (see Figure 5). \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{3}{*}{**Bituminous** **Mixture**} & \multicolumn{5}{c}{**Retroreflectance (\%)**} \\ \cline{2-7}  & \multicolumn{2}{c}{**60\({}^{\circ}\)**} & \multicolumn{2}{c}{**45\({}^{\circ}\)**} & \multicolumn{2}{c}{**0\({}^{\circ}\)**} \\ \cline{2-7}  & **Mean** & **Std. Dev.** & **Mean** & **Std. Dev.** & **Mean** & **Std. Dev.** \\ \hline AUTL-LW & 68.9 & 4.5 & 64.7 & 6.8 & 48.5 & 6.3 \\ \hline AUTL-PB & 8.4 & 1.6 & 7.7 & 1.8 & 6.7 & 0.8 \\ \hline BBTM-LW & 56.0 & 2.5 & 54.1 & 3.6 & 45.9 & 5.7 \\ \hline BBTM-PB & 6.8 & 2.4 & 6.5 & 1.3 & 5.2 & 2.1 \\ \hline AC-LW & 55.8 & 1.9 & 52.2 & 3.6 & 37.9 & 2.7 \\ \hline AC-PB & 6.2 & 2.4 & 5.8 & 1.5 & 5.1 & 1.2 \\ \hline \end{tabular}
\end{table} 
Table 7: Reflectance values in the visible spectrum for different measurement angles. Figure 15: Retroreflection results for the entrance angles of the light source at 60\({}^{\circ}\), 45\({}^{\circ}\), and 0\({}^{\circ}\). Results for the mixtures (**a**) AUTL 8, (**b**) BBTM 8A, and (**c**) AC 16S. Normally, the void content of these mixtures is around 16% for the AUTL, 8% for the BBTM, and 4% for the AC mixtures. However, despite these mixtures usually having quite different textures, it is noteworthy that the white mixtures made with limestone aggregate exhibited a lower-than-usual void content. This is probably due to the low hardness of the limestone, which resulted in higher densification during the compaction process. Consequently, the textures of the different white mixtures are similar (see Figure 18). Nevertheless, the effect of the texture could only be appreciated in the white mixtures, since the reflectance values for the black mixtures are very similar for all the mixture types. In any case, the AUTL mixtures have similar retroreflectance values (slightly higher) to those of the BBTM mixtures. Both types demonstrate values greater than those of the AC mixtures for all the considered entrance angles. For the highest grazing light incident angle (60\({}^{\circ}\)), the retroreflectance values obtained were 68.9%, 56.0%, and 55.8% for the AUTL, BBTM, and AC mixtures, respectively.