# Research on Geometric Design Standards for Freeways under a Fully Autonomous Driving Environment
## 3 Elements of Design
### Stopping Sight Distance
#### 3.3.2 Models and Calculations
_A POLICY on GEOMETRIC DESIGN of HIGHWays and STREETS_ made the following argument [30]: On slippery pavement, when decelerating and braking at a deceleration rate of 3.4 m/s\({}^{2}\), the vehicle can be well-controlled in its own lane, avoiding the impact on vehicles in other lanes. At the same time, most drivers will feel comfortable and safe. Most vehicle braking systems and the tire-pavement friction levels of most roadways are capable of providing a deceleration of at least 3.4 m/s\({}^{2}\) [11.2 ft/s\({}^{2}\)]. The United States takes the deceleration in the stopping sight distance calculation model as 3.4 m/s\({}^{2}\), replacing the longitudinal friction coefficient in the old model. In the current German, American, and Australian alignment design specifications, the longitudinal friction coefficient has been replaced by braking deceleration, and its value is shown in Table 4[27]. In a fully autonomous driving environment, the vehicle's performance and the principle of the braking system have not changed qualitatively. So, it is not recommended to use the friction coefficient to calculate the stopping sight distance. At the same time, combined with the latest research [10; 28; 34], [PERSON] _et al._ still used 3.4 m/s\({}^{2}\) deceleration to calculate the stopping sight distance of autonomous vehicles. When [PERSON] _et al._ calculated the stopping sight distance of L3 autonomous vehicles, the braking deceleration was taken as 3.4 m/s\({}^{2}\); when they calculated the stopping sight distance of L5 autonomous vehicles, they believed that the control performance of the autonomous vehicle would be better than the average level of drivers expected by AASHTO, so they took braking deceleration as 4.5 m/s\({}^{2}\). In addition, [PERSON] _et al._[34] also proposed calculating the stopping sight distance with a braking deceleration of 4.5 m/s\({}^{2}\). The calculation of the stopping sight distance should comprehensively consider the rationality of the calculation method and value of braking deceleration, the uncertainty of deceleration in the braking force rising stage, and the influence of deceleration value on passenger comfort. Based on the above research, according to the braking characteristics of the autonomous vehicle, this paper proposes using average deceleration instead of the friction coefficient to calculate the stopping sight distance. In order to meet the braking safety and the comfort requirements of passengers, the average deceleration is taken as 3.4 m/s\({}^{2}\). \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multicolumn{3}{c}{**Country**} \\ \cline{2-3}  & **U.S.** & **Germany** & **Australia** \\ \hline \multirow{4}{*}{Average longitudinal deceleration (m/s\({}^{2}\))} & \multirow{4}{*}{3.4} & \multirow{4}{*}{3.7} & 2.55 (Comfortable braking) \\ \cline{3-3}  & & & 3.53 (Comfortable emergency braking) \\ \cline{3-3}  & & & 4.51 (Emergency braking) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Braking deceleration in foreign alignment design specifications. The stopping sight distance of autonomous vehicles consists of two parts, the reaction distance and the braking distance, and its calculation is shown in Formula (1) [30]:  
\[S_{\text{st}}=S_{\text{re}}+S_{\text{b}}=\frac{V}{3.6}\cdot t+\frac{(V/3.6)^{2}}{ 2a_{\text{max}}} \tag{1}\]  
where \(S_{\text{st}}\) is the stopping sight distance(m), \(S_{\text{b}}\) is the braking distance (m), \(V\) is the design speed, \(t\) is the system reaction time, taken as 0.5s, and \(a_{\text{max}}\) is the maximum braking deceleration, taken as 3.4 m/s\({}^{2}\).