# Research on Geometric Design Standards for Freeways under a Fully Autonomous Driving Environment
## 4 Geometric Design Standard
### Horizontal Alignment
#### 4.1.3 Minimum Radius of Curve
 \end{tableTo meet the lateral clearance requirements, in a fully autonomous driving environment, a certain lateral clearance of curve shall be provided for the freeway excavation section to ensure that the sensor systems can normally find the way and detect the road condition information ahead, as shown in Figure 5. In this way, the security risk caused by a communication system failure or information transmission delay can be reduced, and the driving speed and traffic efficiency of an autonomous vehicle fleet can be guaranteed. The required radius of a curve can be calculated from the stopping sight distance and the lateral clearance provided by an autonomous driving freeway. The relationship between the three is shown in Formula (4). \[m=R\bigg{[}1-\cos(\frac{28.65S_{t}}{R})\bigg{]}. \tag{4}\]  
Here, \(m\) is the lateral clear distance (m); \(R\) is the radius of a curve at the centerline of the lane within the curve (m); \(S_{t}\) is the stopping sight distance (m), according to the recommended value in Section 3.3. According to Section 4.2.2, the apparent height of an autonomous driving passenger car is taken as 1.85 m. It can be seen from Section 4.3 that the demand for the lane width and width of shoulder will not increase due to the increase of driving speed. The lane width and width of shoulder at different design speeds are not exactly the same. When the design speed is 80-120 km/h, in the most conservative case, the lane width needs to meet the requirement of 3.5 m for large vehicles, and the width of shoulder only needs 1.5 m; when the design speed is 140-180 km/h, in the most conservative case, the lane width needs to meet the requirement of 3.75 m for large vehicles, and the width of shoulder needs 3 m.  
For the above reasons, the lateral clearance provided on the inside of curve is divided into two cases:  
1. When the design speed is 80-120 km/h, the lateral clear distance (in the most conservative case) on the inside of the curve of the excavation section is 3.5 (driveway width)/2 + 1.5 (paved shoulder) + 0.75 (soil shoulder) + 0.6 (side ditch) + 1.00 (crushing platform) + 1.85 \(\times\) 0.75 = 6.9875 m.
2. When the design speed is 140-180 km/h, the lateral clear distance (in the most conservative case) on the inside of the curve of the excavation section is 3.75 (driveway width)/2 + 3 (paved shoulder) + 0.75 (soil shoulder) + 0.6 (side ditch) + 1.00 (crushing platform) + 1.85 \(\times\) 0.75 = 6.9875 m.  
Taking the relevant calculation parameters into Formula 4, the minimum radius of the curve at each design speed of the excavation section can be calculated, and the results are shown in Table 9.  
Figure 5: Demand for lateral clear distance of autonomous driving freeway. The limit minimum radii of a curve with the design speeds of 80 km/h, 100 km/h, and 120 km/h in the Chinese specification are 250, 400, and 650, which are the same as the calculated values in the table. This is because, when the design speed is low, the limit minimum radius is mainly affected by the dynamic characteristics of vehicles. At the same time, the dynamic characteristics of fully autonomous vehicles change little, so the calculated value of the limit minimum radius does not change. However, when the design speed is high, the most important factor affecting the limit minimum radius changes from the dynamic characteristics to the demand for the lateral clearance. * General minimum radius of curve  
The general minimum radius of a curve in the Chinese _Design Specification for Highway Alignment_ is used to ensure driving comfort. Considering the passenger's comfort and experience, referring to Chinese standards, this paper takes the lateral force coefficient as 0.05 and the superelevation value as 6%, which are substituted into Formula (4) and rounded up. The results are shown in Table 11.  
The general minimum radii of a curve with the design speeds of 80 km/h, 100 km/h, and 120 km/h in the Chinese specification are 400, 700, and 1000.