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In a smart traffic light system scenario, the objective is to lower the congestion in the city and optimize traffic flow. The immediate outcome of adopting this approach is the protection of the environment by lowering CO₂ emissions and reducing fuel consumption. Enabling an optimization like this requires the implementation of a hierarchical approach that enables real-time and near real-time operations, as well as analysis of data over long periods of time.

Each intersection in the city represents a component of our system where a smart traffic light application is deployed. The application is in charge of analyzing the collected data from local sensors and CCTV cameras and performs three major tasks: (1) compute the distance of every approaching vehicle in all directions and adapt the traffic light accordingly; (2) monitor pedestrians and cyclists to prevent any accidents, and (3) collect relevant data to help improve the overall system performance. Note that these functionalities require fast response time in case of (1) and (2), the exception being the last functionality (3), which only sends data to a higher layer for further investigation, without waiting for a response.

Another important component of our use case is the global node that creates a control function for each intersection. The key role for a global node is to collect all data from each smart traffic light and determine different commands, such that a steady flow of traffic is maintained. Notice that compared with the time requirements for the tasks deployed at an intersection, the functionality here requires a near real-time response.

The aforementioned hierarchical architecture of our traffic light system benefits from the advantages introduced by the fog computing paradigm. An immediate advantage over the cloud computing paradigm is its capabilities of orchestrating a wide range of distributed devices placed at each intersection. At the same time, it enables devices capable of analyzing data and performing fast response-time actions. Our system can be designed as a four-layer architecture, composed by the sensor layer, a fog device layer present locally at each intersection, another fog layer composed of the global node and the cloud layer. An overview of this architecture is presented in Figure 2.7.