## 2 Introduction of Efficient Pavement Inspection Methods
### Pavement Inspection Methods  
#### 2.2.1 Automated Pavement Inspection Vehicle  
This study introduces the use of an Automated Pavement Inspection Vehicle to enhance the inspection efficiency. The inspection vehicle is shown in Figure 4. Crack measurements are performed using linear laser scanning to capture binary image signals of the road surface; the rutting depth is measured using linear laser scanning combined with a high-speed camera to capture three-dimensional signals at fixed distances; the IRI is measured using displacement sensors combined with vertical accelerometers and distance sensors along the lane direction, with the IRI value calculated from these three devices' results [17]. Additionally, the inspection vehicle is equipped with a front camera to record the inspection conditions, covering a 5 m range in front of the vehicle. The maximum speed limit for inspection is 100 km/h. To avoid laser scattering errors caused by wet pavement, inspections must be conducted on dry surfaces. The accuracy requirements for the inspection vehicle used in this study are shown in Table 7. The study further divides the pavement into single lanes, with each 100 m as a unit (referred to as pavement units) for inspection and analysis.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Maintenance Strategy**} & \multicolumn{2}{c}{**Pavement Inspection Indicators**} \\ \cline{2-3}  & **Pavement Smoothness Indicators** & **Pavement Damage Indicators** \\ \hline \multirow{3}{*}{Provincial Highway Maintenance Units} & \multirow{3}{*}{IRI} & Cracks, surface damage, deformations (ruts and depressions), and \\  & & other damage. \\ \hline Pavement Present Serviceability Index & Smoothness Variation & Cracks, surface repairs, rutting. \\ \hline US Interstate Highway & \multirow{3}{*}{IRI} & Cracks, rutting. \\ Maintenance Strategy & & \\ \hline South Korea Pavement & \multirow{3}{*}{IRI} & Cracks, rutting. \\ Maintenance Strategy & & \\ \hline Japan Pavement Maintenance Strategy & Smoothness Variation & Cracks, surface repairs, rutting. \\ \hline \hline \end{tabular}
\end{table}
Table 6: Comparison of pavement inspection indicators.  
Figure 4: Automated Pavement Inspection Vehicle.  
\begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{1}{c}{**Item**} & \multicolumn{1}{c}{**Accuracy Range**} \\ \hline Distance & Error within \(\pm 0.1\%\) of the actual measurement. \\ \hline Cracks & Identifiable width of 1 mm or more. \\ \hline Rutting & Error within \(\pm 3\) mm of the actual measurement. \\ \hline IRI & Error within \(\pm 5\%\) of the verification unit’s IRI measurement. \\ \hline \hline \end{tabular}
\end{table}
Table 7: Accuracy requirements for the pavement inspection vehicle.  
* Rutting Depth Inspection and Analysis The rutting depth is measured using linear laser scanning technology to measure pavement elevation changes. The specific method is as follows: if the center elevation is higher than the side elevations, the raised center part is used as the reference line; if the center elevation is lower than the side elevations, the raised sides are connected as the reference line. The depth from this reference line to the deepest part of the wheel tracks (D1 and D2) is measured, and the maximum depth is taken as the rutting depth, as shown in Figure 7.
* IRI Inspection and Analysis The IRI is defined based on the average rectified slope, simulating the cumulative elevation difference of a single wheel track as a vehicle travels at 80 km/h over the test road profile, divided by the test road length. This study uses a common inertial profiler for measurement, with the results expressed in meters per kilometer (m/km).