## 5 Discussion
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Angle**} & \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 Small angle curve (\(\alpha\leq 40^{\circ}\)) & 135 & 258.20 & S-M: \(p=0.00551\) & 142 & 315.26 & S-M: \(p=0.00571\) \\ \hline Medium angle curve (\(40^{\circ}<\alpha\leq 70^{\circ}\)) & 120 & 205.11 & S-L: \(p=0.54075\) & 99 & S-L: \(p=1\) \\ \hline Large angle curve (\(70^{\circ}<\alpha\leq 90^{\circ}\)) & 215 & 238.21 & M-L: \(p=0.09731\) & 358 & 309.18 & Sig = 1 \\ \hline \hline \end{tabular}  
* S stands for the small angle curve; M stands for the medium angle curve; L stands for the large angle 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 7: Kruskal–Wallis results of entering curve deceleration and exiting curve acceleration with different angle ranges. drivers tend to operate at higher speeds. Specific warning signs can significantly reduce vehicle speed [46]; thus, speed-reducing signs can be set up at the entrances of curves to warn drivers to slow down. Chevron alignment signs can affect speed control by providing a warning [47]; thus, chevron alignment signs can be laid along the horizontal curve. In terms of the visual improvement of underground roads, the setting of retroreflective rings can enable drivers to maintain good direction perception and reduce reaction time, which improves driving safety [48]. Therefore, an appropriate number of retroreflective rings can be installed on curves. Regarding the sensitivity of acceleration to curve parameters, although the fitted functions obtained in this study have a relatively low R2 value, we only discuss the potential relationship between acceleration and curve radius or curvature based on the graphical representation of the fitted functions. Regarding the relationship between lateral acceleration and curve radius, [PERSON] et al. 's research has concluded that there is a negative correlation between lateral acceleration and curve radius [49]. However, on urban underground roads, when the curve radius is less than 80 m, the lateral acceleration initially decreases and then increases as the radius increases. Conversely, when the radius is greater than 80 m, the lateral acceleration initially increases and then decreases as the radius increases. The reason for the difference between aboveground curves and urban underground curves may be that, as the radius of the curve increases, the linear conditions gradually improve, and the impact of the tunnel environment on the driver's sight distance and vision gradually decreases. In addition, road superelevation also has a certain impact on vehicle acceleration. The distribution characteristics of lateral acceleration of vehicles under different road superele-vations are shown in Figure 14. It can be observed that the lateral acceleration of curves with a superelevation of 2% is generally higher than that of curves with a superelevation of 1.5%. Meanwhile, under the same ultra-high conditions, a larger-radius curve has a smaller lateral acceleration. The distribution characteristics of longitudinal acceleration of vehicles under different road superelevations are shown in Figure 15. Figure 15 reveals that under different superelevation conditions, the deceleration of vehicles entering curves and the acceleration of vehicles exiting curves show little overall difference. This indicates that the impact of superelevation on longitudinal acceleration is relatively small. The main factors affecting longitudinal acceleration are still the radius and angle of the curves. Figure 14: Lateral accelerations at different superelevation.