## 5 Discussion  
According to the risk characterization of driving behavior represented by the G-G diagram, the proportion of driving behavior points on city underground road curves that fall within the safety limits is relatively small. This indicates that the safety risk of curves on urban underground roads is high. A small curve radius increases the risk of driving on curves and the probability of accidents. This finding is consistent with those of a previous study [40]. In addition, higher vehicle operating speeds increase the risk of driving on curves because inappropriate curve speeds are the main cause of collisions, skidding, and rollovers [29, 41]. Regarding the distribution characteristics of risky driving behavior on curves, it was found that the number of dangerous points on the curves increases during the entry phase, remains relatively high during the middle section of the curve, and decreases during the exit phase. This indicates that driving behavior on the horizontal curves of urban underground roads poses the highest risk. However, [PERSON] et al. found the curve entry is the most dangerous location on a curve, followed by the middle section [42]. The possible reasons for this difference are as follows. The curves on urban underground roads are constrained by space, generally resulting in a smaller radius, and the circular curve sections of the curves have the maximum curvature, leading to the highest lateral acceleration. The speed limit on urban underground roads also results in a smaller speed difference between straight sections and curve entries compared with surface roads. As a result, the deceleration during the entry phase of the curve is not too abrupt. Therefore, the driving risk is highest in the circular curve sections of the curves on urban underground roads. In terms of improving curve safety, enhancing the safety performance of critical sections of underground roads can significantly enhance overall driving safety [43]. Therefore, safety improvement measures are proposed based on signage, road markings, and safety facilities, considering the distinct characteristics of risky curves. First, for Curves 1, 5, 7, and 14, which have small radii and poor alignment, the following measures can be implemented to enhance driving safety. (1) The setting of longitudinal deceleration markings and alignment guidance signs can help drivers achieve a smooth transition in curved sections with small radii [44]. Therefore, longitudinal deceleration markings and chevron alignment signs can be set along horizontal curves to improve driving stability. (2) Installing colored antiskid pavement on slopes, curves, tunnel entrances and exits, etc., where accidents are more likely can serve as a good colorful warning and have an antiskid effect [45]. Therefore, a colored antiskid pavement can be set at the curve to improve driving safety. (3) A convex lens can be set in the middle of the curve to improve the poor sight of a small-radius curve. For Curves 2, 3, 5, and 6, which are S-curves, it is recommended to increase the lengths of the straight sections between consecutive curves to enable drivers to adapt better to the transitions between curves. On Curves 4, 12, and 13, vehicle travel data reveal that