# Publish/Subscribe-Middleware-Based Intelligent Transportation Systems: Applications and Challenges
## 1 Introduction  
A U.S. Federal Highway Administration report defines an intelligent transportation system (ITS) as the integration of information technologies, including computing, sensing, and communications, into surface transportation systems, enhancing the efficiency, safety, and environmental sustainability of transportation networks [1]. Also, the United Nations (UN) defines an ITS as encompassing a range of information and communication technologies (ICT) applied to road transport (recent research broaden the definition to all travel modes [2, 3]) to enhance safety, security, and efficiency [4]. These systems offer a range of applications, including significant reductions in traffic congestion, real-time traffic monitoring, and contributions to environmental sustainability and energy conservation [2]. ITS employs technologies, such as sensors, video surveillance, digital mapping, variablemessage signs, and communication networks, to enable dynamic traffic management and support seamless data collection, analysis, and dissemination for data-driven decision making that improves travel time [2]. It is also vital in enabling Automated Driving Systems (ADSs) [5] and integrating Advanced Driver Assistance Systems (ADASs) [6] for enhanced safety, while also providing real-time data crucial for efficient emergency response and informing long-term transportation planning [3]. Figure 1 shows the U.S. Department of Transportation's reference framework for ITS, called the Architecture Reference for Cooperative and Intelligent Transportation (ARC-IT), formerly known as the National ITS Architecture [7]. ARC-IT provides a common framework for planning, defining, and integrating ITS deployments across all modes [8]. The architecture consists of four interconnected views (layers) that capture all aspects of an intelligent transportation system: Enterprise, Functional, Physical, and Communications Views. The Enterprise View defines the organizations, agency roles, and institutional relationships involved in ITS deployment. It focuses on stakeholders and their high-level interactions, such as how a local transportation authority, a transit agency, and state police might collaborate on traffic incident management. The Functional View acts as the application logic layer that outlines abstract functions or processes the system must perform and their logical data flow interactions, like detecting traffic incidents, processing data, and generating traveler alerts. The Physical View translates these functions onto actual physical objects (subsystems and devices) like traffic signal controllers, onboard vehicle units, and control centers, clearly specifying how these components interface and exchange information (center-to-center, center-to-field, vehicle-to-field). Finally, the Communications View details the underlying communication protocols and data standards that support information exchanges among physical entities, ensuring consistent data interpretation across systems through recognized protocol stacks such as IEEE and ISO standards. Figure 1: Architecture Reference for Cooperative and Intelligent Transportation Version 9.3. Furthermore, the communication view effectively defines the networking solutions needed to ensure interoperability among the Physical View Objects [9], directly influencing the overall system performance and flexibility. This paper focuses on the Communications View, examining how a publish/subscribe middleware can unify heterogeneous communication devices and protocols to enable seamless data exchange across an ITS. Adopting a publish/subscribe paradigm, such as the Data Distribution Service (DDS) standard [10], can enhance system performance by decoupling the senders and receivers. Thereby ensuring efficient, asynchronous message dissemination, improving bandwidth and computing resources utilization while enabling real-time data flow [11]. The review also discusses the challenges of integrating publish/subscribe middleware into a heterogeneous ITS environment.