## 4 Results
### Sensitivity of Acceleration to Curve Parameters  
#### 4.3.1 Influence of Curve Radius on Acceleration  
The radius of a curve is an important parameter that describes the geometric characteristics of the curve. Its magnitude affects the centrifugal force acting on the vehicle and the choice of speed by the driver, thereby influencing the changes in the lateral and longitudinal accelerations of the vehicle. Because vehicle stability is worst when the lateral acceleration reaches its peak, the peak lateral acceleration of each participant was extracted while the driver navigated through different curves. The collected data were organized to create a scatter plot of lateral acceleration versus radius, as shown in Figure 12a, along with the corresponding model. Figure 12a describes the relationship between lateral acceleration at the curve of an underground road and the radius of the curve. As can be seen from Figure 12a, the lateral acceleration is minimal at a radius of 115 m and maximal at a radius of 50 m. When R \(\leq\) 50 m, the overall distribution of lateral acceleration is higher. The relationship model between lateral acceleration and curve radius consists of two quadratic functions with opposite openings, represented by fitting curves 1 and 2. Fitting curve 1 shows that when R \(\leq\) 80 m, lateral acceleration decreases first and then increases with increasing radius. Fitting curve 2 shows that when R > 80 m, lateral acceleration increases first and then decreases with increasing radius. These two changing trends are the result of the combined effects of curve radius and vehicle speed. Figure 12b,c, respectively, describe the relationships between entering curve deceleration and radius, exiting curve acceleration, and radius. Figure 12b reveals that drivers perform deceleration more frequently at the entry point of curve 5 (R = 45 m), and less frequently at the entries of curve 9 (R = 60 m) and curve 10 (R = 175 m). Figure 12c shows that drivers accelerate more frequently at the exit of curve 6 (R = 48 m), and less frequently at the exits of curve 2 (R = 200 m) and curve 8 (R = 115 m). It can be found that drivers frequently decelerate during the entry phase of the curve before an S-curve, with fewer instances of acceleration during the exit phase. Conversely, during the entry phase of the curve after the S-curve, there are fewer instances of deceleration, with more frequent instances of acceleration during the exit phase. The changing trend of the fitting curve between deceleration radius and acceleration radius is as follows: with the increase in radius, the deceleration (acceleration) tends to decrease first and then increase. The reason for this changing trend may be that when the curve radius is less than 120 m, the larger the radius, the higher the critical safe speed of the curve, and the smaller the speed reduction required by the driver. Therefore, the deceleration into the curve decreases. However, when the radius is greater than 120 m, the larger the radius and the later the driver reduces speed, leading to an increase in deceleration into the curve. For urban underground road curves, when the radius is small (R < 60 m) or large (R > 160 m), the entering curve deceleration and exiting curve acceleration will be large. The driver's risk model indicates that the smaller the curve radius, the greater the driving risk. Therefore, when it is inevitable to design small radius curves in urban underground roads, it is recommended to ensure that the radius is greater than 60 m as much as possible. Table 6 presents the results of the Kruskal-Wallis test for the deceleration during the entry phase and the acceleration during the exit phase of vehicles at urban underground road curves with different radius ranges. Table 6 shows that there is a significant difference in entering curve deceleration between the extreme small radius curve and the ordinary  
Figure 12: Scatter plot and model of lateral and longitudinal acceleration-radius.