# Research on Geometric Design Standards for Freeways under a Fully Autonomous Driving Environment
## 3 Elements of Design
### Stopping Sight Distance  
#### 3.3.1 Braking Process  
From the perspective of vehicle kinematics, a human-driven vehicle's (HDV's) braking process is affected by the driver's characteristics, so that brake reaction time (reaction distance) occurs. The whole braking process can be divided into three stages: driver perception and reaction stage, braking force rising stage, and full braking stage. Among them, driver perception and reaction stage start from the driver's recognition of a danger signal and lasts until the vehicle starts to brake. The time spent in this stage is T1. This stage also includes three sub-stages. First, the driver visually receives and recognizes the risk factors, and this sub-stage is recorded as t1. Second, the driver decides to and applies the brakes, and this sub-stage is recorded as t2. Third, the brake clearance is eliminated, the vehicle starts to brake, and this sub-stage is recorded as t3. At the end of T1, the vehicle's speed is still initial speed V0, and the vehicle's deceleration is zero. The braking force rising stage starts from the end of T1, the vehicle's deceleration increases, and the vehicle's speed gradually decreases. This stage continues until the vehicle's braking force rises to the max, at which time the vehicle's speed is V1, the vehicle's deceleration is amax, and the time spent in this stage is T2. After that, the vehicle enters the full braking stage. At this stage, the vehicle's speed gradually decreases to the braking end vehicle speed V2, and the deceleration is always maintained at the maximum value amax. The time spent in this stage is T3.  
The driving distance in the driver perception and reaction stage is T1, the driving distance in the braking force rising stage is T2, and the driving distance in full braking stage is T3. S1, S2, and S3 constitute the stopping sight distance ST. The human-driven vehicle's braking process is shown in Figures 2 and 3.  
In a fully autonomous driving environment, autonomous vehicles use sensors, cameras, and other identification systems to identify the road environment, with a wider field of view and higher identification accuracy than traditional drivers. The vehicle's brak  
Figure 3: Schematic diagram of braking process.  
Figure 2: Relationship between braking deceleration and time.  
ing, acceleration, deceleration, steering, and other actions are completed by the on-board computer system. The reaction process of the autonomous vehicles is simple and fast.  
Many scholars at home and abroad have pointed out [26; 27] that with the rapid development of artificial intelligence (AI), deep learning, and supercomputers, the CPU of autonomous vehicles can quickly identify the environment, intelligently make decisions, and efficiently control the vehicle. In the research on stopping sight distance of autonomous driving vehicles, a large number of machine simulation experiments and calculations have shown that the reaction time of autonomous driving vehicles is only about 0.5 s. [PERSON] [10] used a 0.5 s system reaction time to calculate the stopping sight distance of L3 autonomous vehicles. In addition, [PERSON] [28], when calculating the parking sight distance of autonomous vehicles, believed that the system reaction time of the autonomous vehicle from the moment it found the obstacle to the moment when the brakes took effect and started to decelerate was about 0.5 s.  
To sum up, even if advanced technologies such as sensor systems, computers, and AI are used, autonomous vehicles will still take some time to collect road environment information, calculate, make decisions, and eliminate braking clearance. This means that autonomous vehicles also have reaction time and reaction distance. Therefore, the system reaction process cannot be directly ignored when calculating the stopping sight distance of autonomous vehicles.