# Risk Propagation Mechanism and Prediction Model for the Highway Merging Area
## 6 Conclusions  
This study proposes the concept of risk propagation in highway merging areas. A Gaussian-plume-model-based accident impact range model is proposed, which clarifies the impact range of accidents and divides the impact area into three levels. In order to study the risk propagation of each level, a series of simulation experiments were conducted in the highway merging situation. The risk propagation speed was studied through the regional risk index. Therefore, the following conclusions can be drawn. * The simulation results show that the risk of the first impact area remained high throughout the experiment. As the regional risk index in the first impact area decreased, the risk in the second and third impact areas increased. This means that the accident risk spreads from the first impact area to the following areas, which verifies the risk propagation phenomenon of accidents. The risk spreading time from one area to another was affected by traffic flow. Specifically, as traffic flow increases, the risk spread time from the first to second impact areas decreased. This indicates that accidents could spread more quickly in high traffic flow scenarios. The risk spread time from the second to third impact areas also decreased with increasing traffic flow. However, the speed of risk propagation between the second and third impact areas was generally faster than that between the first and second impact areas. * The risk propagation speed of different impact levels is related to the accident location. Specifically, when the accident occurs downstream of the merging area, it takes longer for the risk to propagate from the first-level impact area to the second-level impact area compared to the spread time from the second-level impact area to the third-level impact area. However, when an accident occurs in the merging area, the situation is quite the opposite. This is because when the accident occurs downstream, part of the second-level impact area is in the merging area where there are fewer lanes and a higher likelihood of accidents. In contrast, the downstream area of the merging area has more lanes, larger vehicle gaps and lower vehicle speed. This makes it safer to drive. As a result, the risk takes more time to spread from the first-level to the second-level impact area. Figure 12: The predicted values and actual values of the regional risk index as forecasted by the LSTM model. (**a**) The first impact area. (**b**) The second impact area. (**c**) The third impact area. Furthermore, this study proposed a high-speed highway merging risk prediction model based on LSTM. The model performed well in predicting collision risk. It can be used by researchers and practitioners to predict and monitor highway merging risk accurately. Therefore, different control measures can be taken for different impact areas to achieve precise control and reduce the risk of traffic accidents. However, this study has the following limitations, which need to be improved in future research. The traffic data used in this study were not collected from real highway segments, so the real driving behavior may differ from the simulation results. Therefore, the experiments in this study cannot fully reproduce the real merging situations. In addition, although rear-end and side collisions are the most common collision types in highway merging areas, other collision types should be considered in future research to improve the risk warning mechanism. Conceptualization, Q.Y. ; data curation, B.N. ; funding acquisition, Y.L. ; investigation, Q.Y. ; methodology, Y.L. ; software, B.N. ; validation, B.N. ; writing--original draft, B.N. All authors have read and agreed to the published version of the manuscript. This research was funded by the Fund of the National Engineering and Research Center for Mountainous Highways, grant number GSGZJ-2022-09. This research was also funded by the National Natural Science Foundation of China, grant number 52202419.