# An Emergency Message Routing Protocol for Improved Congestion Management in Hybrid RF/VLC VANETs
## 4 Algorithm Description
### Considered Scenarios  
In our proposed approach, the following scenarios are considered:  
**Front/rear-end collision scenario:** This scenario depicts a collision between two vehicles in the direction of their vehicular axes. Typically, this involves the front end of one vehicle hitting the rear end of the vehicle in front of it. It should be noticed that front/rear accidents are the cause of the most injuries [34], accounting for around 40-50% of all reported vehicle accidents [37]. This makes them the most common collision configuration and scenario for which modeling the emergency notification dissemination is highly desirable.  
Besides, the accident scenario is triggered by the following observation conditions: (a) a sudden impact is detected by the source vehicle, (b) the bumper-to-bumper distance between the source vehicle and the front/back vehicle is less than or equal to 0, indicating a contact, (c) the velocity of one or both vehicles decreases significantly upon the impact, and (d) the vehicles come to a stop shortly after the collision once the momentum is dissipated.  
On another note, the accident clearance time is predicted based on several factors such as (a) the number of cars involved in the accident, (b) the number of lanes blocked, (c) the blocked lanes are unidirectional or bidirectional, (d) the traffic flow conditions, (e) the location of the accident, i.e., whether it is in an intersection, a regular road, in the middle of the road or on the sides.  
Now, in the event of an accident, the source vehicle analyzes the real-time link qualities and simultaneously broadcasts the warning message on all its assigned RF/VLC channels and frequencies. Then, the receiving vehicles in the source's zone re-forward the message using a VLC to vehicles behind them whenever possible. They will relay the message using the RF when the VLC links are obstructed. This information is also transmitted to the associated RSU to enable further broadcasting across the zones as needed. The vehicles that received the emergency message immediately switch their radios to the dedicated high-priority modes prepared by the RSU schedules. This prepares them to receive the subsequent emergency broadcasts with increased ranges. By leveraging the adaptive RF/VLC resource assignments using our proposed algorithm, the emergency notifications are efficiently propagated throughout the affected area in a manner that overcomes the dynamic propagation conditions.  
**Congestion scenario:** This scenario involves detecting the congestion by gathering information about the positions and speeds of the vehicles using traffic view techniques [38]. Congestion is confirmed when any of the following factors has occurred: (a) the number of vehicles within a certain region exceeds a predefined threshold, (b) the average vehicle speed is below a certain threshold that is also far below the posted speed limit, or (c) the distance between vehicles within a certain region I below a certain threshold.  
When a congestion situation is determined based on the above factors, the vehicles behind the source are informed about the congestion, its location, the start time, the current duration, and the predicted clearance time. Based on Table 1, the type of blocked lanes and the accident location are the two most influencing factors used to determine the predicted clearance time.