## 4 Results
### Driving Risk Model  
#### 4.2.1 Driving Risk Model and Its Distribution Characteristics  
Vehicle acceleration, deceleration, and angle are the main factors influencing driving behavior. The variation in longitudinal acceleration is related to the application of the brake and accelerator pedals. Changes in lateral acceleration are influenced by the amplitude and speed of steering. When driving on curves on urban underground roads, drivers assess the risks of upcoming curves based on their experience and perform different driving maneuvers to maintain vehicle stability. In this study, the risk of driver behavior on different curves of urban underground roads was evaluated using G-G diagrams, referring to the methodology proposed by [PERSON] [21] for constructing a driving-style graph model. The abscissa in the figure represents the lateral acceleration Ay, where Ay > 0 in the first and fourth quadrants and Ay < 0 in the second and third quadrants. An Ay of \"+\" indicates a right turn, and an Ay of \"-\" indicates a left turn. The ordinate represents the longitudinal acceleration Ax, where Ax > 0 in the first and second quadrants and Ax < 0 in the third and fourth quadrants. An Ax is \"+\" when the driver takes acceleration measures, and it is \"-\" when the driver takes braking measures. In Figure 9, the enclosed area within the closed curve represents safe driving behavior, whereas the area outside the curve represents dangerous driving behavior. Therefore, whether the current driving behavior is risky was determined based on the position of the vehicle lateral and longitudinal accelerations in the graph at a given moment. In addition, owing to the complex alignment of urban underground roads, the characteristics of more entrances and exits, and the radical driving styles of some drivers, vehicles still face driving risks, even if they are being driven within the safety limits defined by the G-G diagram. Therefore, an indicator Ms can be introduced to evaluate the level of aggressiveness in the driving behaviors of drivers within the safe range. This indicator is defined as:  
\[\text{Ms}=|\text{OE}/\text{OF}| \tag{3}\]  
Figure 9: G-G graph of driving behavior. The formulas defining the safe boundaries for quadrants one and two are as follows:  
\[\left\{ \begin{array}{ll}0\leq A_{x}\leq 0.15&A_{y}=0.15\\ A_{x}=0.509A_{y}^{2}+2.351A_{y}+2.841&-2.5<A_{y}<-0.15\\ A_{x}=2.5&-0.15<A_{y}<0.15\\ A_{x}=0.509A_{y}^{2}-2.351A_{y}+2.841&0.15<A_{y}<2.5\\ 0\leq A_{x}\leq 0.15&A_{y}=2.5\end{array} \right\} \tag{4}\]  
The formulas defining the safe boundaries for quadrants one and two are as follows:  
\[\left\{ \begin{array}{ll}-0.15\leq A_{x}\leq 0&A_{y}=-2.5\\ A_{x}=-0.446A_{y}^{2}-2.395A_{y}-3.349&-2.5<A_{y}<-0.15\\ A_{x}=-3&-0.15<A_{y}<0.15\\ A_{x}=-0.446A_{y}^{2}+2.395A_{y}-3.349&0.15<A_{y}<2.5\\ -0.15\leq A_{x}\leq 0&A_{y}=2.5\end{array} \right\} \tag{5}\]  
To differentiate the different driving behaviors, a new dashed region can be added to the G-G diagram (Figure 9). This region can be defined with Ms = 0.5, serving as a boundary to delineate the various driving behaviors. When 0 \(\leq\) Ms \(\leq\) 0.5, it represents driving behavior safety. When 0.5 \(<\) Ms \(\leq\) 1, it indicates dangerous driving behavior. Figure 10 shows the G-G diagram representing the driving behavior of each driver on curves of urban underground roads (partial G-G diagrams). Figure 10a reveals that in the extreme small radius curves, when driving on independent curves 1, 7, and 14, most of driver behavior points fall into the driving danger zone, indicating the presence of risky behaviors on these three curves. Among them, curve 1 has the most points falling in the danger zone, and these points are distributed far from the closed curve, indicating the highest level of safety risk. When driving on continuous curves 5 and 6, curve 5 has more behavior points falling in the danger zone than curve 6, indicating that curve 5 has a significantly higher driving risk than curve 6.