# Connected Vehicles: Technology Review, State of the Art, Challenges and Opportunities
## 3 Applications
### Safety Applications  
The advancements in safety systems, such as active and passive systems, have been growing rapidly to mitigate many fatal traffic accidents and to help drivers and passengers survive. Active safety systems assist drivers in avoiding accidents through in-advance audible and visual alarms or by assisting drivers in managing their vehicles, such as through anti-lock braking systems. These systems are considered preventive systems, which help drivers avoid accidents before they occur [65; 66]. Whereas, passive safety systems help minimize accidents' impacts on the drivers and passengers in case of accidents; these include seat belts and air bags [66]. The integration of wireless communication in the automotive industry has led to many innovative active-system wireless-based applications among vehicles and between vehicles and infrastructures. On that account, these applications or cooperative safety driving applications provide traffic-related data to drivers beyond their perception, and, hence, they mitigate traffic accidents and reduce congestion [65].  
CV applications demand low latency, efficient, fast, and reliable broadcasting protocols for delivering messages in single-hop and multi-hop communications. However, some of the common issues associated with dissemination algorithms include the hidden node problem, the unavailability of feedback. and the high rate of collisions in the broadcast messages [67]. Several forms of message broadcasting can take place in vehicular environments. This includes beacon messages that send a vehicle's status and situational data in high frequency and small packets in a periodic manner. Even though beacon messages are considerably important in cooperative awareness applications, it suffers from some pitfalls such as latency, network flooding, and a lack of location preciseness [11]. Whereas, event-driven messages transmit critical event-based information, for instance, in cases where safety critical events should be noted; therefore, these messages will require minimum delay and a lower collision probability rate [11; 68].  
Safety-related data such as basic safety messages (BSMs) help drivers to have a full real-time awareness of the surrounding environment, since they constantly share their speed, acceleration, and direction and other relevant safety data. Another form of safety traffic data is event-based data, as shown in Figure 3, where a vehicle ahead of the way is able to notify other vehicles about incidents that lie ahead or an alarming situation, such as emergency electronic brake light ahead warning or forward collision avoidance [69].  
Safety applications are time-critical applications, which require real-time data to be delivered with low latency, reliability, and a high transmission speed. However, to enhance latency and performance, challenges with respect to data management and analysis should be addressed. Even with the high cloud scalability and its low cost, solutions such as, increasing the computational power are not feasible in terms of data management challenges [11]. Moreover, safety applications will require thorough analysis in terms of safety evaluation and modeling [70; 71; 72]. In the following subsections, some of the most popular CV safety applications are discussed.  
Figure 3: Post-crash safety warning scenario.