# Hidden Markov Model-Based Dynamic Hard Shoulders Running Strategy in Hybrid Network Environments
## 1 Introduction
### Literature Review  
The existing research on the HSR strategy have focused on identifying the key indicators that determine the opening and closing of the hard shoulder. The following categories of indicators are commonly considered:  
1. Traffic flow: Urban highway congestion is primarily caused by the traffic flow exceeding road capacity during peak hours. Therefore, many scholars consider traffic flow to be the key index for implementing the HSR strategy, which temporarily expands the road capacity by opening the hard shoulder. [PERSON] et al. [31] found that opening the hard shoulder when traffic exceeded a certain threshold helped alleviate congestion caused by heavy traffic flow. [PERSON] [32] also explored the effect of the dynamic opening of the hard shoulder on road bottlenecks, using traffic flow as a starting point. The hard shoulder increases the traffic capacity of bottleneck sections.  
2. Safety: Extensive studies have been conducted on the safety impact of opening the hard shoulder [5; 33; 34; 35; 36], and the primary purpose of the hard shoulder is to serve as an emergency lane for vehicles that have broken down. If the goal is to maximize highway capacity, the hard shoulder will be in use throughout the simulation. However, it is important to consider the needs of accidents and not overuse the hard shoulder. In that case, [PERSON] et al. [6] introduced a safety weight to incorporate safety factors into the design of the hard shoulder open strategy. Other researchers believe that HSR can be used as a supplementary measure to VSL. The hard shoulder is only opened when VSL is unable to handle excessive traffic [19; 20] in order to balance the negative impact on safety indicators.
3. Speed: Some researchers have explored the possibility of the open threshold of the hard shoulder from the angle of speed change characteristics. [PERSON] et al. [37] introduced the concept of traffic breakdown as a threshold for hard shoulder opening. [PERSON] et al. [38] argued that breakdown occurs when traffic flow switches from free to congested, which is frequently accompanied by continuous oscillation, resulting in large-scale congestion. Consequently, it becomes necessary to identify traffic breakdowns and manage them in advance. The definition of traffic breakdown has not been uniformly quantified. Many researchers have different definitions of breakdown based on the changing characteristics of road sections from free to congested, establishing a speed drop threshold or a density rise threshold [39; 40]. [PERSON] et al. [37] defined the critical vehicle speed and provided a method for calculating the drop threshold. However, most traffic breakdown studies have only focused on traditional human-driving scenarios, and it appears that few studies have examined mixed traffic flow scenarios.  
As different users place different emphasis on safety indicators and traffic, there are many possible weighting relationships between safety indicators and traffic, which will also have a great impact on the hard shoulder opening policy. Considering that the speed characteristics of the road section can reflect the change in traffic flow and the possibility of accidents, this paper mainly considered the speed factor when designing the dynamic HSR strategy.