## 2 Reviewing Road Traffic Management Systems  
Today, the traffic management systems offer capabilities that can be utilized to reduce the road congestion, improve the response times in case of car accidents (related to delaying the emergency services), as well as enhance the travel experience of commuters. Typically, the TMS logic comprises four complementary phases, as depicted in Figure 1. The key building block of the represented chain is data sensing and gathering functionality, in which heterogeneous road monitoring equipment [20] measures the important traffic-related parameters (e.g., traffic volume; speed; and occupancy of the road segments) over certain time intervals. Further, the measured data is forwarded to the central TME; the detected events are immediately reported over the deployed wireless communications networks (e.g., cellular systems). Then, the obtained data feeds are processed (aggregated) in the second block known as the data fusion, processing, and aggregation (DFPA). The following block, named the data exploitation (DE), processes the knowledge from DFPA and computes the (i) optimal routes; (ii) short-term traffic forecast; and (iii) supplementary road traffic statistics. The final block, termed the service delivery (SD), distributes the resultant knowledge to the end users/commuters (e.g., drivers, private companies, emergency services, etc.). It is important to note that the utilization of features delivered by the TMS is not limited to drivers and authorities, but may also significantly contribute to the economic growth, safety of citizens, and national security. Currently, the deployed systems and technologies are still unable to acquire accurate traffic parameters and can miss real-time reports of accidents that occur along the road networks, especially in developing regions. Operating more sophisticated equipment to ensure highly-accurate assessment of traffic flows and timely detection of emergency situations may seem to be a preferred solution. However, this choice remains limited due to constraints in financial resources available to support the needed dense deployment and maintenance of such costly equipment, which may lead to a lack of flexibility [9]. Therefore, alternative cost-efficient and agile solutions are needed to offer better management of road traffic. To overcome some of the aforementioned constraints, modern TMSs aim at adopting innovative technical approaches to exploit more advanced solutions and thus monitor the evolving road networks more closely. In particular, this allows for exploiting the short-term predictions based on current traffic volumes to identify the impeding bottlenecks and make more informed decisions on how to reroute traffic, change lane priorities, and modify traffic lights sequences most appropriately. Further, contemporary traffic management services need to also provide visual tools for displaying the real-time traffic situation related to the (i) location of traffic jams; (ii) accidents; (iii) congestion levels for each of the road network segments; and (iv) estimated travel time across the road network. Using this information, responsible authorities will have a better real-time perspective on the road network and might in turn conduct more adequate traffic management as well as achieve more efficient reactions in case of emergency and road accidents.