## 3 Dataset  
Based on the measured deflections, the E\({}_{\text{AC}}\) are obtained through backcalculation procedures. Classical backcalculation procedures require the assumption of the initial modulus of elasticity at the beginning of the calculation, on which the convergence further depends. A wrong choice of the initial modulus of elasticity often leads to a calculation that is not sufficiently accurate. Other problems that affect the accuracy of the calculation include the accuracy of measurements, the number and thickness of the pavement construction layers, as well as the temperature of the asphalt layers [36; 37; 38; 39]. In order to form a model for predicting the elastic modulus of asphalt layers of flexible pavement structures, it is necessary to form a set of data of the measured deflections and temperature, which contain a sufficient amount of information. Based on this data set, the modulus of elasticity is calculated using classic backcalculation procedures. Starting from the defined goals and the analysis of the research plan and program, a database that first needs to be thoroughly examined and processed is chosen. Data analysis and processing mean that when deflections are measured on the primary and secondary roads and highways, the recurring points that represent extremes and significant deviations are removed. In this way, they do not affect the data analysis. In addition, 462 pieces of information were collected for analysis, 438 of which were taken for network training, which amounts to 95% of the total amount of data. Network testing was performed based on 23 pieces of information, which is 5% of the total amount of data. The statistics of the data used in the modeling are shown in Table 3.  
The models were modeled by the ANN, SVM and BRT methods using the MATLAB software package. The input variables are deflections \(d_{0}\), \(d_{300}\), \(d_{600}\), \(d_{900}\), \(d_{1200}\), \(d_{1500}\), \(d_{1800}\), i.e., the measured deformations at vertical distances of 0, 300, 600, 900, 1200, 1500, 1800 mm from the center of the loading plate and the temperature of the upper surface of the asphalt layer. The deflections were mostly measured in spring and autumn in the early morning hours, except for one section on the highway, which was measured in the summer period in the morning hours. ELMOD 6 software was used for the backcalculation. Three methods for calculating the asphalt modulus can be selected in this software. They include the finite element method, linear elastic theory and method of equivalent thicknesses. In this paper, the linear elastic theory was used. Since in the Republic of Serbia there is no recommendation for using these three methods, British standards are used [40]. The output variable is the modulus of elasticity of the asphalt layers (E\({}_{\text{AC}}\)) expressed in MPa. \begin{table}
\begin{tabular}{l c} \hline \hline
**BRT Parameters** & **Values** \\ \hline Learning rate, \(\
u\) & 0.03 \\ \hline Boosting stages & 190 \\ Leaf size & 12 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: BRT algorithm parameters. The deflections were measured based on the load which ranges between 48.63 and 50.89 kN. Due to such a small range, the load is considered constant, and hence it is not taken as an input variable. The thickness of asphalt layers was measured at intervals of 1 km and for that reason was not taken as an input variable. According to British standards [40], the bearing capacity of the structure is divided into three sections, according to the module values:  
* modules up to 3000 MPa--poor bearing capacity overall;
* modules from 3000 MPa to 7000 MPa--there are damages in some places;
* modules above 7000 MPa--good bearing capacity overall. As a result of the aforementioned division, in this research, the models were modeled with modules of up to 7000 MPa of the maximum value. Figure 6 shows the percentage values of the number of the current elastic modulus E\({}_{\text{AC}}\), within the given ranges.