## 4 Results
### Sensitivity of Acceleration to Curve Parameters
#### 4.3.2 Influence of Curve Angle on Acceleration  
The curve angle is another important parameter that describes the geometric characteristics of a curve. The size of the curve angle has an influence on the vehicle driving characteristics, driver line of sight, and operational requirements. At larger curve angles, drivers must start steering and decelerating earlier to maintain vehicle stability and safety. Therefore, the size of the curve angle affects the magnitude of the longitudinal acceleration of the vehicle. Simultaneously, drivers must use a larger steering angle to control the steering wheel and maintain stable circular motion, thereby influencing vehicle lateral acceleration. Figure 13a describes the relationship between lateral acceleration at the curve of an underground road and the curve angle. The figure shows that the lateral acceleration is at its minimum when the angle is 50\({}^{\circ}\) and at its maximum when the angle is 54\({}^{\circ}\). When the angle is less than or equal to 50\({}^{\circ}\), the overall distribution of lateral acceleration is relatively high. The model of the relationship between lateral acceleration and the curve angle is composed of two quadratic functions with opposite openings, represented by fitting curves 1 and 2. Fitting curve 1 shows that when the angle is less than 50\({}^{\circ}\), lateral acceleration tends to increase and then decrease with the increase in the curve angle. Fitting curve 2 shows that when the curve angle is greater than 50\({}^{\circ}\), lateral acceleration tends to decrease and then increase with the increase in the curve angle. Figure 13b,c, respectively, describe the relationships between entering curve deceleration and curve angle and exiting curve acceleration and curve angle. The figures show that the longitudinal acceleration and deceleration change similarly with the curve angle and the radius, showing a trend of decreasing first and then increasing with the increase in angle. For urban underground road curves, when the angle is small (\(\alpha<40^{\circ}\)) or large (\(\alpha>70^{\circ}\)), the entering curve deceleration and exiting curve acceleration will be high. Driver risk models indicate lower driving risks on curves with small angles; hence, for urban underground road curve designs, it is recommended to keep the curve angle \(\alpha<70^{\circ}\). Table 7 provides the results of the Kruskal-Wallis test for vehicle deceleration during the entry phase and acceleration during the exit phase of curves in urban underground roads across different angle ranges. As shown in Table 7, there is a significant difference between the entering curve deceleration of the small angle curve and the large angle \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Radius**} & \multirow{2}{*}{**n**} & \multicolumn{2}{c}{**Entering Curve Deceleration**} & \multirow{2}{*}{**n**} & \multicolumn{2}{c}{**Exiting Curve Acceleration**} \\ \cline{3-3} \cline{5-6}  & & **Mean Rank** & & **Parameter** & **Mean Rank** & **Parameter** \\ \hline Extreme small radius curve (R \(\leq\) 85 m) & 235 & 222.80 & E-G: \(p=1\) & 368 & 302.46 & E-G: \(p=0.16334\) \\  & & & Sig = 0 & & Sig = 0 \\ \hline General radius curve (85 m \(<\) R \(\leq\) 150 m) & 114 & 229.99 & E-O: \(p=0.0153\) & 122 & 267.63 & E-O: \(p=0.41328\) \\  & & & Sig = 1 & & Sig = 0 \\ \hline Ordinary radius curve (R \(>\) 150 m) & 121 & 265.36 & G-O: \(p=0.138\) & 110 & 330.41 & Sig = 1 \\ \hline \hline \end{tabular}  
* E stands for the extreme small radius curve; G stands for the general small radius curve; O stands for the ordinary radius curve. Sig = 1 indicates that the mean difference is significant at the 0.05 level; Sig = 0 indicates that the mean difference does not exist at the 0.05 level.  
\end{table} 
Table 6: Kruskal–Wallis results of entering curve deceleration and exiting curve acceleration with different radius ranges. curve. There is a significant difference between the entering curve deceleration of the medium angle curve and the large angle curve.