## 4 Results and Discussion
### Factor Analysis Based on PDPs  
In this study, the single-factor PDPs of the equivalent number of actions of a standard axle load, annual average temperature, number of freeze-thaw cycles, freezing index, annual precipitation and road section maintenance times were analyzed, and the results obtained are shown in Figure 10.  
1. The effect of the equivalent number of actions of a standard axle load on pavement roughness, as shown in Figure 10a. As the equivalent number of actions of a standard axle load approaches 0, IRI is high, but as the equivalent number of actions of a standard axle load increases, the IRI value decreases and stabilizes. And ultimately, as the equivalent number of actions of a standard axle load rises, IRI again shows an increasing trend. This is because as the traffic load increases, the pavement improves initially due to compaction. But excessive traffic loads can exacerbate the damage to the pavement structure and lead to deterioration in pavement roughness.
2. The effect of the number of freeze-thaw cycles on pavement roughness, as illustrated in Figure 10b. As the number of freeze-thaw cycles increases, the IRI value gradually increases. It indicates that the freeze-thaw cycles have a significant negative effect on pavement roughness. During the freeze-thaw cycle, the pavement material undergoes repeated expansion and contraction due to temperature changes, which ultimately leads to cracking and damage to the pavement, thus affecting the roughness.  
Figure 10: Partial dependence plots. (**a**) ESAAL. (**b**) FREEZE_THAW. (**c**) TEMP. (**d**) FREEZE INDEX. (**e**) PRECIPITATION. (**f**) MAINTENANCE.  
3. The effect of annual average temperature on pavement roughness, as depicted in Figure 10c. With the increase in temperature, IRI showed a gradual decrease, and IRI was higher under low-temperature conditions. As the temperature increased, the IRI gradually decreased, and after the temperature increased to a certain extent, IRI increased. This shows that pavement materials become brittle at low temperatures and are prone to cracking and breaking, resulting in uneven pavements. As the temperature rises, the material becomes more flexible in warmer climates and is able to maintain better roughness. In high-temperature environments, pavement deterioration problems can occur, resulting in uneven pavement surfaces. 4. The effect of freezing index on pavement roughness, as presented in Figure 10d. The influence curve of the freezing index on the IRI shows some fluctuation. When the freezing index is low, the IRI value has a slight tendency to decrease, while as the freezing index increases, the IRI value gradually increases. This shows that a higher freezing index can exacerbate pavement damage caused by freeze-thaw cycles, leading to a reduction in pavement roughness. 5. The effect of annual precipitation on pavement roughness, as described in Figure 10e. The increase in precipitation has resulted in a fluctuating upward trend in IRI values. This is because the moisture penetrates into the pavement structure, leading to a softening of the road base and a reduction in the strength of the pavement. It will exacerbate the unevenness of the pavement. 6. The effect of road section maintenance times on pavement roughness, as demonstrated in Figure 10f. The frequency of pavement maintenance is found to be positively correlated with IRI values, with higher levels of maintenance resulting in higher IRI values. This may be due to the observation that more frequently maintained road sections tend to have higher levels of damage. It can be hypothesized that road sections that are subject to higher levels of maintenance may have higher IRI values and therefore require more frequent maintenance.