# Optimal Path Routing Protocol for Warning Messages Dissemination for Highway VANET
## 1 Introduction  
According to World Health Organization research [1], road accidents claim the lives of almost 1.3 million people every year around the world. VANETs offer a viable approach for avoiding such incidents. VANETs are mobile ad hoc networks [2] that can operate with a diversity of models, such as the Vehicle-to-Infrastructure (V2I) model and Vehicle-to-Vehicle (V2V) model, as depicted in Figure 1. Advanced communication protocols such as IEEE 802.11p and IEEE 1609 send information between vehicles in the fundamental VANET architecture. VANETs encompass a diverse set of utilizations divided into two categories, i.e., comfort and safety applications. Cooperative collision prevention and traffic updates are examples of safety applications.  
In contrast, comfort applications provide value-added services, including entertainment, travel time prediction, path identification, environmental protection, and road conditions [3]. The prevention of traffic accidents is the most crucial application among these. Safety applications in VANETs mostly rely on warning message (WM) broadcasting. When a vehicle senses a hazardous incident, it transmits WMs to the surrounding vehicles, allowing them to take appropriate action to avoid traffic accidents [4; 5]. However, on the other hand, unnecessary broadcasting causes several serious problems, including road accidents, redundancy in transmission, and high latency [6].  
Therefore, a rapid and reliable path for WMs is critical, as latency and packet drops during WMs delivery can cause large-scale collisions among vehicles [6]. When a sender vehicle is ready to send WMs, it may face two scenarios. To begin with, if there is just one path to the target vehicle, it is unavoidable to take that path. Secondly, choosing the optimal route with the least latency and packet drop becomes critical if multiple paths to the target vehicle are available. As a result, selecting the following hop, also known as the optimal forwarder (_Of_), is critical during the delivery of WMs.  
Greedy routing methods [7] consider parameters such as the present positions of intermediate relay nodes and their distances from the target node. When vehicles are static, such parameters are valid. In VANETs, the greedy routing protocols [8] chose the next-hop in a target vehicle direction, which is a good strategy in uni-directional road traffic. However, direction-based greedy protocols' performances worsen in bi-directional road traffic when a next-hop is picked based on its current location in the direction of the target vehicle. The distances among vehicles change in a bi-directional highway environment because nodes can move at high speeds in both directions. Hence, due to the constant changes in topology, these vehicles leave and enter the transmission ranges with one other regularly. As a result, a path chosen at a specific time step does not remain fixed indefinitely. It may alter at later time-steps somewhat for transmitting even a single warning message. This alteration may happen concerning a decreased or increased number of hops that affect the latency appropriately. Furthermore, paths are broken, and new paths are generated at routine intervals, potentially resulting in network partitions. Therefore, all warning messages routed across the damaged paths are dropped. These aspects lead to high packet losses, delays in WMs transmission, and reduced throughput of the networks.  
We present a unique cluster-based approach, OPRP, to disseminate WMs in highway VANETs, addressing the abovementioned difficulties. OPRP considers our mobility parameters when choosing a CH to improve cluster stability and reduce communication overhead. Only a cluster head can disseminate the WMs among its cluster members. Moreover, CH transmits WMs across adjacent cluster heads and increases network efficiency.