# Advances of UAVs toward Future Transportation: The State-of-the-Art, Challenges, and Opportunities
## 3 State of the Art of Practice and Research
### Current Practice and Research
#### 3.1.6 Future Intelligent Transportation Systems (ITS)  
With the world population forecasted to be doubled by 2050, there is an increased interest in innovative smart city development. For designing smart cities, there is a need to fully integrate the information and communication technology solutions and their trends. UAVs can be integrated tighter with the vision of creating smart cities. An intelligent transportation system is one of the key elements for developing a smart city. With the continuous advancements in ITS, efforts have been made for the worldwide deployment of smart vehicles. As the number of autonomous and connected vehicles increases, many new elements and services will be enabled. Some of the possible ITS applications of UAVs and the potential challenges for UAV-enabled ITS in smart city development are identified by [PERSON] et al. [14]. These include flying accident report units, flying roadside units, and flying police eyes. One of the most investigated areas is the development of a UAV-enabled traffic monitoring system. To date, UAVs have been used to collect traffic information in image or video formats, but still, the full-fledged usage of UAVs in real life for road traffic management is in the early stages of development [20, 54, 80]. [PERSON] et al. performed a survey on the variety of UAV-based traffic monitoring systems that were proposed by several researchers [81]. [PERSON] et al. systematically reviewed the research studies conducted in traffic monitoring and analysis by UAVs until the year 2016. They provided a detailed step-by-step framework in this domain [54]. [PERSON] et al. presented a review of existing literature until the year 2016 about UAV applications in the transportation sector, including traffic monitoring and management [82]. Up to the year 2018, most of the existing UAV-based traffic monitoring systems only used one UAV with a fixed trajectory to extract vehicle information. [PERSON] et al. proposed a smart traffic monitoring system that can use UAV with 5G technology [55]. [PERSON] et al. proposed a road traffic monitoring system using multiple cooperative UAVs [53]. It was concluded that the performance of the proposed multiple UAV system was better than that of the fixed UAV trajectory traffic monitoring system in terms of coverage rates and event detection rates. In another recent research study, [PERSON] and [PERSON] conducted an experimental research study to monitor urban congestion with a swarm of UAVs [56]. In the study, traffic streams were recorded in a congested urban setting using a UAV swarm to investigate critical traffic phenomena. Another major area of the current application is how to retrofit the existing transportation infrastructure to accommodate UAVs for future ITS. [PERSON] et al. investigated the UAV docking station placement problem for ITS [83]. In their following research, a generic management framework of UAVs for ITS was developed [84]. Figure 7 depicts an example case scenario for UAV-enabled ITS, adapted from Ref. [14]. Figure 7: An example case scenario for UAV-enabled ITS [14]. UAVs are introduced in ITS as a tool for traffic monitoring that requires coordination between UAVs and vehicles. UAVs could also be utilized as Roadside Units (RSUs) to support communications by ensuring reliable internet access, emergency response, and other safety notifications [85]. Compared to fixed RSUs, UAVs have an additional degree of freedom due to their three-dimensional mobility. Fixed ground-based RSUs only detect incidents in areas within their specific range, and they are subject to placement restriction constraints. In comparison, due to the flying abilities of UAVs, challenges such as area coverage size and road network restrictions can be overcome. Hence, UAVs' wireless channel qualities and communication ranges can be superior to that of the fixed RSUs [84]. Both vehicle-to-vehicle (V2V) and vehicle to infrastructure (V2X) communications could be ensured by using UAVs.