###### Abstract
[13]. The technical contribution of UAV integration into ITSs offers several advantages. UAVs equipped with cameras and sensors can monitor traffic from an aerial perspective, complementing ground-based sensors and cameras. They provide real-time traffic updates, detect congestion, and monitor accidents more efficiently in areas that are otherwise hard to cover using traditional ITS infrastructure like road cameras or sensors. UAVs can quickly reach incident sites, providing high-resolution imagery and live video feeds to ITS control centers. This supports quicker detection of and response to road accidents, natural disasters, or infrastructure damage. UAVs collect data from large geographical areas, feeding them into ITS databases for advanced analytics. These data include traffic flow, weather conditions, and infrastructure health. UAVs also enhance communication by acting as aerial relays in areas where ground-based communication networks are sparse or temporarily disabled. UAVs are used for environmental monitoring tasks like air quality checks and noise level assessments, integrating these data into ITSs to maintain sustainable transportation networks. However, despite these advantages, significant limitations must be addressed before large-scale implementation occurs. To provide a holistic view of the complexities and opportunities in developing a UAV-based transportation system, a three-layered framework is proposed to analyze the interdependencies among the layers, along with the cybersecurity measures and expertise required to build a smart transportation system. Key issues include UAV security and privacy concerns, challenges with automated UAV navigation, and the limited battery life of current UAVs [14,15]. To successfully integrate UAVs into future transportation systems, it is crucial to have a comprehensive understanding of their advancements, challenges, and applications in this sector. Although UAVs have been explored in various domains, their full potential in transportation remains underexplored. Existing research often focuses on specific aspects rather than providing a holistic overview. Therefore, this paper comprehensively reviews recent developments in UAVs within transportation systems using a structured three-layered framework. The framework addressescritical aspects of UAV integration into ITSs, including (1) UAV technical specifications, (2) communication networks for responsive traffic monitoring, and (3) advancements in data collection and processing. This framework bridges the gap between theoretical advancements and practical applications by demonstrating how the integration of UAV technologies can enhance transportation systems. Moreover, it identifies future research and development opportunities in this field. Although the authors propose a three-layered framework, it ideally serves as a structured overview of previous and current fragmented efforts organized within each subtopic or layer toward a successful UAV-enabled ITS application. Organizing the content in this manner provides a comprehensive perspective on the advancements in UAV-enabled ITSs, guiding future research directions while acknowledging the historical context that has shaped the field. The objectives of this review paper are as follows: (1) to present a detailed summary of UAV-enabled ITSs within the three-layered framework, (2) to analyze the current state of each aspect of the framework, and (3) to identify future research directions for developing robust UAV-enabled ITSs. The rest of the paper is organized as follows. Section 2 proposes examining UAV-enabled ITSs under a three-layered framework. The results and analysis of the current fragmented efforts related to the proposed framework are discussed separately for each layer in Sections 2.1-2.3. Additionally, the discussion is separated into two parts, with Section 3 exploring the current research areas and application of UAVs in transportation and Section 4 examining the challenges, opportunities, and future research directions in the realm of UAV-enabled ITSs.