## 4 Roughness Measurement Using a Straightedge  
The straightedge method for the pavement rut depth measurement [84] was selected for the data collection from the real gravel pavement. The proposed static method is based on measuring the gap between the road surface and a constant length (3.0 m) beam placed along the track corresponding to a vehicle's left-side wheels. In this study, the straightedge method for measuring gravel pavement longitudinal roughness was adopted, which is usually characterised by higher roughness. Therefore, a small correction in measurement was made by adding 20 mm height spacers (\(h_{\text{spacer}}\)) at the ends of the beam (Figure 2). An additional spacer was used in the cases when the end of the beam was over pavement dent.  
Figure 1: IRI computation and evaluation [39]: (**a**) Golden car model and its parameters; (**b**) IRI sensitivity to wave number.  
The actual height of elevation and dent is determined by subtracting added spacer height (20 mm) considering each measured vertical distance, which is the clearance between the bottom of the straightedge and the pavement surface at corresponding elevation and dent:  
\[h_{\text{el}} =h^{\prime}_{\text{el}}-20, \tag{11}\] \[h_{\text{dent}} =h^{\prime}_{\text{dent}}-20, \tag{12}\]  
where \(h^{\prime}_{\text{el}}\) and \(h^{\prime}_{\text{dent}}\) are the measured height of elevation and dent, respectively (mm); \(h_{\text{el}}\) is the actual height of elevation or clearance between the bottom of the straightedge and the pavement at corresponding elevation (mm); \(h_{\text{dent}}\) is the actual depth of dent or clearance between the bottom of the straightedge and the pavement at corresponding dent (mm).  
The first stage of data post-processing included the inversion of data on the measured pavement elevation. Thus, the negative values of pavement dent were obtained. The absolute depth (amplitude) of the first wave \(\Delta_{\lambda_{1}}\) (Figure 2), meaning the difference between dent depth and the average of neighbouring elevations, is expressed by the formula:  
\[\Delta_{\lambda_{1}}=h_{\text{dent,1}}-\frac{h_{\text{el,0}}+h_{\text{el,1}}} {2}. \tag{13}\]  
The absolute depth (amplitude) of the second and next waves are as follows:  
\[\begin{split}\Delta_{\lambda_{2}}=h_{\text{dent,2}}-\frac{h_{ \text{el,1}}+h_{\text{el,2}}}{2},\\ \vdots\\ \Delta_{\lambda_{N}}=h_{\text{dent,N}}-\frac{h_{\text{el,N}-1}+h_{ \text{el,N}}}{2}.\end{split} \tag{14}\]  
A straight section of rural road, 160 m in length, with gravel pavement was selected for this study. Pavement roughness was measured considering the different states of the pavement. The first state (Pavement State 1, PS-1) means that the gravel pavement measurement was carried out two days after pavement maintenance using the blading technique without adding gravel material to the existing road surface. This state corresponds to the best-expected quality and driving conditions. The second state (Pavement State 2, PS-2) is described as a middle-quality gravel road, three weeks after pavement maintenance and the start of intensive road deterioration. The third state (Pavement State 3, PS-3) corresponds to the worst pavement quality, when maintenance is expected.  
After blading, large grains were observed on the gravel's flat surface; however, it did not have a significant influence on vehicle response because of tyre enveloping properties. The formed ripples were noticeable on the surface before maintenance (worst condition), and almost no individual free grains were found.  
Figure 2: The measurement of gravel road roughness applying a straightedge: (**a**) schematic view, 1—straightedge of 3 m in length; 2—added spacer; 3—pavement elevation; 4—pavement dent; (**b**) measurement of gravel pavement.