# An Emergency Message Routing Protocol for Improved Congestion Management in Hybrid RF/VLC VANETs
## 2 Related Work
Clustering incurred sizable signaling costs and lacked fault tolerance. Also, it incurs high signaling overhead maintaining dynamic groups. Position-based routing depended too heavily on position heuristics without reaction to congestion, which may fail under changing topologies or high loads. Much of the existing research focuses on RF systems for VANETs with little incorporation of the emerging VLC communication technology. Only a few authors suggested various hybrid VLC-RF systems for VANETs. Mainly, in [30] the authors compared the performance of a standalone VLC system using various modulation schemes. Then the performance of that VLC system is compared with that of a hybrid system which proved to offer long-distance transmission, link reliability, and an increase in the received information. Moreover, little work has focused on optimizing hybrid systems specifically for emergency scenarios at intersections and congested roadways. Most evaluate general connectivity performance but do not examine techniques for maximizing the reliability of alert dissemination under disruptive conditions. There is a need to investigate coordination mechanisms, congestion-aware adaptation, and multi-hop forwarding methods for resilient emergency messaging. This work develops advanced algorithms for emergency message routing and congestion management in vehicular networks. For emergency messaging, it leverages the directional nature of Visible Light Communications (VLC) to optimize the propagation of warnings throughout the affected area. A partitioning scheme assigns dedicated radio frequency RF and VLC resources to zones, minimizing interference. Optimization ensures emergency broadcasts simultaneously utilize all assigned channels for maximum reach. Store-and-forward routing extends connectivity between zones when needed. For congestion handling, a centralized approach is taken where RSUs continuously monitor traffic flows to build real-time models. Clearance times are simulated under various strategies to score impacts on travel time, emissions, and user satisfaction. Strategies are classified by severity and vehicles vote on preferred options. Adaptive signals prioritize the selected strategy while maintaining coordination across adjacent areas. This data-driven technique aims to resolve congestion more efficiently through collaborative optimization of signaling, routing, and traffic throughput. While hybrid RF-VLC systems show potential for addressing interferences and blockages faced by singular technologies, gaps remain in optimizing them specifically for timely emergency response:  
* Prior work focuses on general connectivity but does not fully develop the coordination protocols required to ensure reliable message dissemination even when direct links are intermittent or disrupted. * Existing approaches lack dynamic adaptation to changing traffic flows and topologies, limiting their responsiveness under real-world congestion. * Bottlenecks at intersections where incidents are most critical are not comprehensively addressed. Scalable solutions optimized for these dense merge areas are needed. * Proactive mitigation of emergent jams through predictive clearance strategies coordinated across network partitions has seen little attention. Several key gaps our work aims to address include:  
* Reliability of emergency alert dissemination: We investigate techniques like coordinated VLC/RF retransmissions and aerial-assisted routing to maximize reliability under high congestion/disruption conditions not fully addressed before. * Responsiveness to dynamic traffic environments: The dynamic mapping of topology via real-time beaconing allows our system to adapt broadcasts/routing based on varying loads/topologies, improving emergency response time compared to static schemes. * Resilience against intermittent link interruptions: Leveraging both short-range localized VLC and longer-range multi-hop RF routing provides redundancy that enhances dissemination even when direct connections are blocked.