# Research on Geometric Design Standards for Freeways under a Fully Autonomous Driving Environment
## 3 Elements of Design
### Decision Sight Distance  
The function of decision sight distance is to ensure that drivers who have not received the export notice information can change lanes in time, slow down, and then leave the main line before the exit of the interchange, parking area, or service area. Decision sight distance can prevent drivers from missing the exit due to his/her own negligence and ensure that vehicles travel on the expected route. When calculating decision sight distance, _A POLICY on GEOMETRIC DESIGN of HIGHWAYS and STREETS_ believes that pre-handling time should be longer than reaction time in the stopping sight distance calculation model, so that drivers have enough time to deal with road traffic conditions. As an important design element to ensure that the vehicle follows the prescribed route and avoids drivers changing lanes or reversing near the exit, the application and efficacy of decision sight distance in an autonomous driving environment will make a big difference.  
In a fully autonomous driving environment, with the cooperation of on-board sensors, CPU, and high-precision maps, autonomous vehicles can fuse, correct, and label information collected by multiple sensors. By positioning, marking signs, and road condition information, autonomous vehicles can achieve centimeter-level positioning [37; 38]. According to relevant research [38; 39], there are two types of route-planning methods for autonomous vehicles: global route planning and local route planning. Accurate and flexible local route planning can ensure that autonomous vehicles travel according to the optimal route planned between starting and ending points. At the interchange exit, global route planning tells autonomous vehicles where to go and which ramp to take off the main line. Local route planning guides autonomous vehicles to avoid obstacles, overtake, and change lanes by outputting control signals such as trajectory, speed, and steering angle. Two route planning methods cooperate with each other. This successfully solves the problems of drivers misjudging traffic information, making wrong decisions, not preparing to leave the main line in advance, and missing the interchange exit in the traditional driving environment.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Design Speed (km/h)**} & \multicolumn{6}{c}{**Downgrade (\%)**} \\ \cline{2-7}  & **0** & **2** & **3** & **4** & **5** & **6** \\ \hline
140 & 250 & 260 & 265 & — & — & — \\ \hline
120 & 185 & 195 & 200 & 205 & & \\ \hline
100 & 130 & 135 & 140 & 145 & 150 & \\ \hline
80 & 85 & 90 & 95 & 95 & 100 & 100 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Stopping sight distance of automatic driving truck on downgrade.  
To sum up, in a fully autonomous driving environment, vehicles can successfully identify the interchange exit without decision sight distance. At the exit, vehicles only need to decelerate and drive smoothly into the offramp without braking. Therefore, it is suggested that the geometric design of freeways at the exit of the interchange should no longer be bound and limited by decision sight distance and only needs to meet the stopping sight distance of autonomous vehicles.