# Efficient and Safe Strategies for Intersection Management: A Review
## 3 Efficiency
### Emergency Vehicle Priority  
This section proposes a particular vehicle: The emergency vehicle. Statistics show that emergency vehicles are more prone to accidents than other road users when performing tasks, especially at intersections, as is shown in Figure 11. Statistics on the German rescue service mission show that there are more than 14 million missions per year, which is equivalent to having tens of thousands of emergency vehicles perform tasks every day. Each task is carried out under tremendous time pressure because the local response time controls the maximum time difference between the incoming call and the arrival of the rescue team [65]. The emergency vehicle preemption system allows emergency vehicles such as fire fighting trucks and ambulances to request and receive green traffic signal indications as they approach an intersection [66]. Emergency Vehicle Preemption (EVP) provides a green belt along the route at isolated and coordinated signal intersections, interrupting normal traffic signal timing operations. EVP allows the EVs to pass without stopping or waiting at the intersection, which may reduce travel time and reduce conflict with other vehicles in the system. [PERSON] et al. [67] study the priority of EV signals from different directions and necessarily pass the same intersection in a specific period time, and propose a multi-agent EV signal priority control system based on multi-agent. The phase agent and management agent are imported in the system, and the fuzzy logic theory is used to realize the internal logic of various agents and the coordination mechanism between modules. The Dynamic Preemption Algorithm (DPA) [68] is based on the idea of adjusting the traffic signal cycle at each intersection along the road to ensure sufficient green light to allow the emergency vehicles to pass through the intersection with minimal delay without stopping. The algorithm must determine the emergency vehicle at a particular location and then decide the exact seconds at which the preemption must begin in the traffic signal phase period based on the Global Positioning System (GPS) information received from the EV. The decision of position must strike a balance between two essential requirements: One is that the minimum travel time for an emergency car should approximately equal to its theoretical free travel time, the other is that the additional delay for other vehicles should reach the minimum or be slight enough at the same intersection. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Algorithm** & **Real-Time** & **Robustness** & **Time Complexity** \\ \hline State lattice [49] & 85\% & ++ & ++++ \\ \hline EB [50] & 75\% & + & ++++ \\ \hline A* [54] & 88\% & ++ & ++ \\ \hline RRT [57] & 79\% & ++ & ++ \\ \hline Q-learning [60] & 90\% & ++++ & ++ \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Algorithms performance of trajectory planning. The algorithms comparison of the aboved are shown in Table 7. Many of the developed preemption strategies are based on a single intersection. These strategies rely on local detection and clearing intersections one by one, and the signal preemption process cannot be initiated before an emergency vehicle is detected; the inherent delays at the intersection are unavoidable. This signal preemption strategy can only be initiated after the detection of the emergency vehicles, which often leads to inevitable intersection delay. [PERSON] et al. propose a signal preemption control method named Timed Colored Petri Nets (TCPN) to reduce the delay of emergency vehicles at intersections [69]. TCPN is used to establish a traffic flow model, traffic signal display, phase switching model, and traffic signal switching control model. An urban transportation network model consists of three sub-models. An emergency vehicle preemptive optimization control system is designed, including monitoring subsystem, phase time determination subsystem, and phase switching control subsystem. [PERSON] et al.