### Pavement Maintenance Strategies in Different Countries  
In recent years, maintenance units in various countries have drawn on past experiences, selecting significant pavement indicators for inspection and establishing decision-making grading intervals for each indicator. Maintenance decisions are made based on these grading intervals. This section evaluates the pavement inspection and decision-making methods in different countries. * The American Association of State Highway and Transportation Officials (AASHTO) collected 10 years of pavement observation data and formed a panel of pavement experts to evaluate these data. The panel identified parameters and performed mathematical analysis to establish the Present Serviceability Index (PSI), which is used in the AASHTO Guide for Design of Pavement Structures for pavement thickness design and overlay decision analysis. The PSI is calculated using the example in Equation (1) [4], which involves parameters such as pavement smoothness, surface cracking, repair level, and rutting. \[\text{PSI}=5.03\ -\ 1.91\ \text{log}(1+\text{SV})-0.01\sqrt{\text{C}+\text{P}}-0.2 1\overline{\text{R}\text{D}}^{2}\] (1) where: \[\text{SV}=\text{Smoothness Variation}\] \[\text{C}+\text{P}=\text{Cracking and Patching Rate}\] \[\text{RD}=\text{Rutting Depth}\]
* In 2012, the United States passed the MAP-21 funding bill, which includes pavement maintenance strategies proposed by the Federal Highway Administration (FHWA) [5]. This strategy involves inspecting the IRI, crack rate, and rutting depth, setting grading intervals for these indicators into the \"Good\", \"Fair\", and \"Poor\" categories, as shown in Table 1.  
* In South Korea, pavement inspections for the IRI, crack rate, and rutting depth are conducted, and the National Highway Pavement Condition Index (NHPCI) is calculated based on these results, using the example in Equation (2) [6]. [PERSON] analyzed data from South Korean highways [7], setting maintenance thresholds for the IRI, crack rate, and rutting depth at a 95% confidence level for different warranty periods (3, 5, and 7 years), as shown in Table 2. \[\text{NHPCI}=\frac{1}{(0.33+0.003\times\text{X}_{\text{CR}}+0.004\times\text{X}_{ \text{RD}}+0.0183\times\text{X}_{\text{IRI}})^{2}}\] (2) where: XCR = Crack Rate (%) XRD = Rutting Depth (mm) XIRI = IRI (m/km)  
* In Japan, pavement inspections focus on the IRI, crack rate (including the surface repair rate), and rutting depth. The Ministry of Construction's Civil Engineering Research Institute uses historical data to develop the Maintenance Control Index (MCI) through multiple regression analysis, with the calculation shown in the example in Equation (3) [8]. Additionally, the Metropolitan Expressway Company in Japan develops maintenance strategies based on the IRI, crack rate (including the surface repair rate), and rutting depth inspection results [9]. The criteria for determining the inspection results are shown in Table 3, where the pavements are categorized into six grades (A, B1, B2, B3, C, and D) based on the average rutting depth, maximum rutting depth, average crack rate, and maximum crack rate. Maintenance status is broadly divided into three categories: \"Good Condition\" for grade D, \"Track Required\" for grades C and B3, and \"Maintenance Required\" for grades B2, B1, and A. \[\text{MCI}=10-1.48\text{C}^{3}-0.29\text{D}^{0.7}-0.47\text{G}^{0.2}\] (3) where: C = Crack Rate (%) D = Rutting Depth (mm) \(\sigma\) = Smoothness Variation (mm) \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Grading** & **IRI (m/km)** & **Crack Rate (\%)** & **Rutting (mm)** \\ \hline Good & \textless{}1.5 & \textless{}5 & \textless{}5 \\ Fair & 1.5–2.7 & 5–10 & 5–10 \\ Poor & \textgreater{}2.7 & \textgreater{}10 & \textgreater{}10 \\ \hline \hline \end{tabular} (Source: Compiled from FHWA-HIF-17-022, 2016 [5]).  
\end{table} 
Table 1: Grading intervals for various indicators. * In summary, most countries currently inspect the IRI, crack rate, and rutting depth of pavements.