# Advances of UAVs toward Future Transportation: The State-of-the-Art, Challenges, and Opportunities
## 1 Introduction
The existing ground transportation infrastructure has become more congested each year with an increasing number of cars and reduced road capacity levels [10]. It was reported that in 2019, an average of drivers in the United States (US) lost about 99 h due to traffic congestion [11]. Many technologies and logistics industries have started to shift goods by employing UAVs to ease the unsolvable traffic congestion issues. UAVs are beginning to emerge and are slowly integrated into the current transportation infrastructures. UAVs can drastically change and become an integral part of the future transportation sector in the near future. The progress and advancement in UAV's capabilities have led to their usage in many different areas of the transportation sector. The global UAV-assisted logistics and transportation market is proliferating, with projected market size of about 11 billion dollars by 2026, from about 5.3 billion dollars in 2019 [12]. UAVs have been employed for logistic delivery purposes in medical, e-commerce, and backyard delivery [13]. The rapid growth of UAV usage is primarily observed due to the emerging e-commerce market and growing customer expectations of faster delivery times. UAVs are also being integrated into other transportation sectors like urban planning and management, traffic monitoring, and disaster response or relief. With more and more vehicles being automated nowadays, in the future, UAVs will play a crucial part in developing a fully automated transportation system and assist in the form of the automated field support team, traffic police, road surveys, and rescue teams [14]. Figure 1: The global market size and forecasted global economies for UAVs in 2024 [6]. Despite their various potential applications, there are some limitations and concerns regarding UAVs use. Some of the issues include intrusion of privacy, UAV collisions due to loss of control, hacking, and other security issues [15,16]. These issues need to be overcome before full-fledged active deployment in real-life transportation systems becomes a reality [16]. To employ UAVs successfully in future transportation systems, it is crucial to have comprehensive knowledge about the current and potential advancements of UAVs, their challenges, and opportunities of usage in the transportation sector. Thus, this paper provides concise analyses and reviews of some recent advances of UAVs and their roles in the current and future transportation systems. The objectives of this paper are to (1) provide the state-of-art applications of UAVs in the future transportation sector, (2) analyze the advantages and disadvantages of using UAVs, and (3) identify some future research directions in order to achieve a robust UAV-enabled intelligent transportation system (ITS) in the future. Figure 2 represents the literature review flowchart and the structure of this paper. \(n\) denotes the number of research articles and transportation-related project reports reviewed and considered during the literature review process. Note that the numbers presented are the numbers after a rigorous screening. The rest of the paper is organized as follows: Section 2 lays down the basics of UAVs and explains the fundamentals. Section 3 reviews the recent state of the art of application of UAVs in the transportation sector and identifies additional potential applications, challenges, and opportunities. Section 4 discusses the future research directions in the domain of UAV-enabled future transportation. Lastly, the concluding remarks are given in Section 4.