# Publish/Subscribe-Middleware-Based Intelligent Transportation Systems: Applications and Challenges
## 4 Application of Publish/Subscribe Middleware in ITS
### Transportation Mode
#### 4.2.2 Land Transportation  
Land transportation is the most widely used mode of transportation. Its popularity is due to its accessibility, flexibility, affordability, and the developed infrastructure of roads and highways. This peculiar characteristic makes ubiquitous ITSs challenging. Figure 11 is the distribution of vehicles in the United States [69]. It shows that 91% of vehicles are cars, light trucks, and motorcycles. Heavy-duty vehicles and boats make up less than 10%, and trains and planes account for under 1% of the total. Therefore, ITSs with real-time functionality are necessary for smart land transportation due to the diversity and size of this transportation category. Bikes and Bicycles  
[PERSON] et al. [70] developed an intelligent support system for cyclists that is context-based and makes use of information collected from various on-board bicycle sensors as well as a smartphone that is in front of the cyclist. It creates a real-time model of the physical space to determine the cyclist's capabilities and the surrounding context of the e-bike. The system makes recommendations to the cyclist and controls the level of electrical assistance given based on the information provided. It uses a publish/subscribe mechanism for information sharing in the Smart Space, allowing for efficient communication and collaboration between different system components. Also, the system monitors the e-bike's engine's electrical assistance level, which assists in lower power usage and preserves battery life. The system combines a smartphone, sensors, and e-bike data for intelligent cyclist support, enhanced cyclist experience, and improved safety. However, some users may find that using this system is complicated to adapt, and the effectiveness of the system depends mainly on the capabilities and reliability of the smartphone. Heavy and Light Vehicles  
Another application of ITS in land transportation is event monitoring. [PERSON] et al. [71] proposed an Event Data Recorder (EDR) for both heavy and light land vehicles. The system records visual and textual data from the car and uploads it to a server. The authors used  
Figure 11: US vehicle distribution by transportation mode (2021) [69]. the MQTT protocol to manage the EDR configurations, such as time intervals for data recording. The server is a broker and publisher, and the EDR subscribes to synchronization information. However, the architecture assumes reliable connectivity between vehicles and the central server, which may not hold in remote or low-coverage areas. Additionally, relying solely on MQTT without integrated security or fault-tolerance mechanisms poses risks to data integrity. Also, using a single centralized broker further introduces a critical single point of failure, which could disrupt the entire system in the event of a crash or attack. [PERSON] et al. [72] proposed a participatory sensing system for a dynamic public transport feed using an Extensible Messaging and Presence Protocol (XMPP)-based publish/subscribe middleware. Its primary objective is to provide precise arrival times for public vehicles at specified stops. It combines two different kinds of data to achieve this accuracy. (1) Static data includes fixed schedules and routes of public transportation. It is the predetermined information about when vehicles are supposed to arrive at stops according to their regular schedules. (2) Real-time crowd-sensed data gathered from the passengers using their mobile devices. It provides real-time updates on vehicle arrival times, accounting for delays or schedule changes. By combining these two, the system ensures that it publishes reliable and up-to-date information to the customers subscribing to the feed and reflects the real-time situation of the public transportation system. Similarly, [PERSON] et al. [73] developed TrafficInfo, a participatory sensing-based live public transport information service using XMPP-based publish/subscribe middleware.