# Publish/Subscribe-Middleware-Based Intelligent Transportation Systems: Applications and Challenges
## 4 Application of Publish/Subscribe Middleware in ITS
### Infrastructure
#### 4.1.2 Vehicular Ad Hoc Networks (VANETs) Infrastructure
for vehicular safety applications, significantly improving network efficiency compared to traditional ITS communication models. However, VANETs have one challenge that middleware alone cannot solve: the high infrastructure cost of deploying RSUs across large geographic areas to ensure reliable communication. For example, the simulation setup for VANET in Luxembourg deployed 2247 RSUs at junctions, with an additional 191 edge RSUs placed at 500-m intervals on highways to ensure complete coverage [56]. Such a dense RSU deployment translates to substantial initial investment costs, including installation, maintenance, and connectivity expenses. A cost-effective alternative is leveraging existing infrastructure (e.g., traffic lights and streetlights) already strategically placed at intersections by enhancing them with embedded systems and lightweight publish/subscribe middleware. The Intelligent Traffic Light (ITL) event based publish/subscribe system demonstrates how traffic signals equipped with wireless communication and a real-time publish/subscribe model can relay safety messages to vehicles, reducing the need for additional RSUs [58]. By enabling vehicles to subscribe to traffic light alerts, ITL ensures real-time dissemination of critical warnings, such as red light running alerts and required deceleration notifications. Also, separating publishers from subscribers allows for greater system scalability and a more dynamic network topology. This approach minimizes infrastructure expansion costs while enhancing vehicular communication and road safety. Alternatively, the vehicles can cooperate and provide service to one another using V2V communication. [PERSON] et al. [59; 60] proposed a brokerless service-oriented middleware solution called Cooperation as a Service (CaaS) designed for VANET and Vehicular Sensor Networks (VSNs) using content-based routing (CBR) for intra-cluster communications and Delay- and disruption-Tolerant Network (DTN) routing for inter-cluster communications. By leveraging V2V communication, it aims to provide free services, increase the quality of communication, and significantly enhance the driving experience by facilitating more efficient and service-oriented communication between vehicles without adding extra infrastructure. At its core, CaaS enables vehicles to cooperate, sharing valuable information for all road users. The architecture utilizes a hybrid publish/subscribe mechanism, where drivers express their interests in specific services, and vehicles that have subscribed to the same service cooperate in providing the necessary information to one another. By leveraging V2V communication protocols, CaaS ensures a seamless exchange of information. Also, the architecture boasts high delivery ratios. However, the system has a high complexity of implementation and maintenance. Also, the system requires at least two cars for communication, which is only possible in cities but seldom happens on highways. Researchers use publish/subscribe middleware in VANET to improve the reliability, scalability, and responsiveness of the ITS. The solutions reviewed in this section use COPSS for content-oriented safety alerts [56], CaaS for decentralized V2V service exchange [59; 60], and the ITL system for infrastructure-integrated messaging, demonstrating how custom middleware can address specific challenges such as network congestion, infrastructure cost, and real-time coordination. Notably, all the middleware implementations in this section are custom-developed to meet the unique demands of their respective use cases rather than relying on standard platforms like DDS or MQTT. This section emphasizes the need for domain-specific middleware architectures in VANETs, where factors such as mobility, intermittent connectivity, and deployment constraints require specialized design.