# Publish/Subscribe-Middleware-Based Intelligent Transportation Systems: Applications and Challenges
## 2 Methodology
The SLR used the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA [17]) framework. PRISMA is a set of evidence-based guidelines designed to improve the transparency and quality of reporting in systematic reviews and meta-analyses [17]. It provides a structured 27-item checklist and a flow diagram to ensure comprehensive reporting of methodology, data selection, and synthesis processes. PRISMA helps researchers enhance reproducibility, minimize bias, and improve the reliability of systematic reviews in various scientific disciplines. Figure 3 shows the PRISMA SLR flowchart for this review. The identification stage searches for all publications that may help answer the research questions. The flowchart shows that we used the Scopus database [18] to find the papers for this review. We selected the Scopus database for this review due to its extensive  
Figure 2: A Venn diagram of publications of interest with respect to the search keywords. coverage of peer-reviewed literature across multiple disciplines relevant to intelligent transportation systems, its support for reproducible search strategies, robust analytics tools, and comprehensive indexing of publications from all major and reputable publishers. We searched for all publications with the keywords \"publish/subscribe\" and \"transportation\" within their titles, abstracts, or keywords. The search term did not include middleware because the Venn diagram in Figure 2 shows that the 15 papers it contributed to are at the intersection of all search terms. Thus, they will be included even without it in the search term. The search yielded 87 publications from 2003 to 2024, with no duplicate entries. However, publications from 2003 to 2008 did not meet our inclusion criteria, which we describe shortly. As a result, the final set of reviewed papers spans from 2009 to 2024. The screening stage removed 4 entries because they are conference proceedings titles, not publications, resulting in 83 papers for further analysis. At the eligibility stage, we applied well-defined inclusion (in Table 2) and exclusion criteria (in Table 3) to filter the publications systematically. Included papers must fulfill all inclusion criteria and none of the exclusion criteria. The inclusion criteria in Table 2 ensured that only relevant and high-quality papers were selected. Specifically, we included papers focusing on publish/subscribe-based middleware implementation in transportation, addressing specific applications, challenges, or security concerns. We considered all scientific publications, including peer-reviewed articles, conference papers, and review papers, ensuring a comprehensive analysis. The selected publications were required to utilize publish/subscribe-based middleware as a communication channel in transportation applications and fall within the publication timeframe of 2009 to 2024 to capture historical developments and emerging trends. We used this time frame because all the publish/subscribe middleware in transportation papers in the Scopus database fall within it. The exclusion criteria for this research are shown in Table 3. We eliminated duplicate publications to maintain diversity and originality. We also excluded non-peer-reviewed sources like blogs, websites, and opinion pieces. This criterion ensures academic integrity because unreviewed articles may contain biased, wrong, or false information. Figure 3: PRISMA dataflow diagram for this review. We also removed retracted, incomplete, inaccessible, or invalid papers. Additionally, this work considers only publications in English, excluding those in other languages. Lastly, this review excludes all articles that discussed middleware without a direct application in transportation. These criteria excluded 18 papers based on their titles, as they did not align with the research scope. Furthermore, 25 papers were removed after thoroughly reviewing their abstracts or the complete content, as they did not meet the inclusion criteria.