# Research on Safe Following Distance on an Expressway Based on Braking Process Analysis
## 2 Related Work  
There are many reasons for causing traffic accidents, among which, the failure to maintain a safe following distance accounts for a large proportion. [PERSON] et al. analyzed 1521 expressway traffic accidents in mountainous areas from 2012 to 2017 and found that one of the most common factors involved in serious accidents in the daytime was the failure to maintain a safe distance [2]. [PERSON] et al. analyzed the causes of road traffic accidents on foggy days and believed that the main reason was that the drivers failed to allow a sufficient following distance [3]. There are many reasons for having a dangerous following distance, such as driving speed, driver conditions and road conditions. The method of solving these problems and reducing the occurrence of traffic accidents is of particular importance. In 2012, [PERSON] et al. designed a rear-end collision warning system based on Zigbee technology, which can compare the actual distance measured by radar with the safe distance and provide early warning information for drivers [4]. In 2013, [PERSON] et al. proposed an improved safety distance model (SDM) based on vehicle-to-vehicle communication technology, which realized real-time data acquisition and real-time workshop communication and could effectively improve the efficiency of a vehicle anti-collision system [5]. In 2014, [PERSON] et al. proposed a self-optimizing PID model based on fuzzy control which could assist a car in following another car at a safe distance, and this model was established on a wireless sensor network sharing information for perceiving different stauses of the traffic stream [6]. [PERSON] et al. established a practical calculation model for safe gaps. In addition, they used Matlab to simulate three different safe gaps under dry pavement [7]. In 2018, [PERSON] pointed out in his paper that if the available sight distance is less than the required sight distance, the possibility of the driver completing a certain action will be reduced [8]. In the article, he also proposed a process through which the sight distance of the highway can be evaluated. In 2013, [PERSON] et al. designed an effective, intelligent traffic safety distance control system for foggy days [3]. In 2022, [PERSON] et al. proposed a new design method for a recommendation system which can suggest the safe speed on the road, and the system can suggest the safe speed for overtaking vehicles to prevent collisions by taking into account the distance from the previous vehicle [9].The rational design of roads is also of great significance to the safe driving of vehicles. [PERSON] et al. discussed the safe driving conditions on the highway and obtained the relationship among the design length of the vertical curve, the stopping sight distance and the slope algebra, and gave treatment suggestions [10]. If the conditions of a section of road cannot meet the requirements of safe sight distance, traffic engineering control measures can be taken to ensure traffic safety as much as possible. [PERSON] et al. established a parking sight distance calculation model based on driver and vehicle performance and recommended the parking sight distance of different models through actual data verification [11]. [PERSON] et al. established the line-of-sight calculation model by analyzing drivers' characteristics and road alignment and other factors [12]. [PERSON] et al. for the first time, used the friction coefficient between the tire and the ground instead of the vehicle speed reduction in an emergency as the main calculation parameter in the calculation of sight distance [13]. [PERSON] et al. established a new parking sight distance model based on GPS data [14]. [PERSON] et al. proposed the mathematical model and calculation method of 3D sight distance through parametric analysis of road section [15]. According to the coordinate equation and cross section parameters of road median line, [PERSON] et al. put forward the highway 3D sight distance test technology [16]. [PERSON] et al.